TOC

• Author

• About the Author

• Last Updated 

• Status

• Disclaimer 

• Overview 

Solar System General

• Off-grid VS grid-tied 

• What does off-grid mean? 

• Government Solar Subsidies 

• Solar Tariffs 

• Cash vs Debt vs Lease Purchase 

• Roof vs Ground Installation 

• How to Connect Solar Electricity to House 

• Electricity Rates by State 

• Electricity Rates California 

Solar System Savings

• Payback time 

• Energy used by appliances 

• My energy use 

• Low energy appliances 

• Price per Watt vs Cost per kWh 

Solar System Design

• Solar System 

• Solar Panels 

• Charge controller 

• Battery 

• Inverter 

• Wire Size 

• Operation 

• Solar System Wiring Diagrams

Solar System Appendix

• Design Tips 

• Design Questions 

• Solar System Tips 

• Solar System Future Trends

• Solar System Abbreviations 

• Solar System Formulas 

Author

Brad Brown

About the Author

Brad Brown is a Silicon valley computer programmer that lives in California.

Last Updated

11/10/2025

Status

This document is currently in development and still being updated until further notice here.

Disclaimer

General

Although, I try to make this document accurate, I'm not responsible or liable for any errors. I'm not responsible or liable for any problems or damages that you might encounter with your solar system if you use any information from this document. In addition, any work you do with solar system is done at your own risk.

Warning High Voltage

You can encounter high voltages from solar systems, including solar panels, charge controllers, batteries, and inverters, that can be dangerous and deadly so always take proper safely precautions when handling high voltages. This document is not intended to teach you how to work with high voltages. In addition, any work you do with high voltages is done at your own risk.

Overview

The DIY Off-grid Solar System Guide shows how to create a DIY off-grid solar system for use in the United States paying particular attention to saving money. 

I live in California with the highest electricity rates in the nation so to save money I need to get a solar system. In 2024, my rates in California are about 47 cents a KWH compared to Las Vegas rates of about 12 cents a KWH; therefore my rates are about 4 times more than Las Vegas. To put that into perspective, if I run a 1000 watt space heater for 2 hours it will cost me almost $1 and it would cost someone in Las Vegas about $0.24 cents.

I want to make sure when I buy a solar system that I'm buying what I need so I don't waste my money. 

I'm going to focus on the lowest cost solar systems under $1000, that are capable of running most 120V home appliances. I'm not going to focus on large whole house solar systems but you can use the information here to build as powerful of a solar system as you want.

I'm going to use this document when I build my solar system and I'm providing it free to others to help them build their solar systems.

The requirements for my solar system are:

• Off-grid: With an off-grid solar system you generally won't need permission from the government or any permits.

• Saves Money: I would like a payoff period of three years or less.

• Expandable: I don't want to spend a lot of money all at once so when possible I want a system that is upgradable.

This guide won't cover how to do the following topics but I will mention them so you know they exists.

• Grid-tied solar system.

• Government subsidies.

Off-grid vs Grid-tied

A solar system can be either off-grid or grid-tied or sometimes referred to as on-grid. In my opinion, it is better to have a DIY off-grid solar system for the reasons given in this section.

Off-grid solar system

An off-grid system can be something you DIY or you could pay someone to setup the off-grid solar system for you.

• Inexpensive solar system cost: Typically installed as a DIY project but you could hire a professional to install it for you if you want. The solar system could cost as little as about $500 for a very small system.

• Inexpensive maintenance and repair cost: If you installed the solar system yourself then you can do maintenance and repairs yourself to save money.

• No net metering: Can't sell electricity back to the grid.

• No Permit Required: In general, you don't need a permit for an off-grid solar system but you might need a permit in some cases depending on what you want to do. For example, if you want to install a transfer switch to your breaker box a permit might be required but installing a transfer switch is optional.

• No Power Outages: Often a grid-tied solar system can't provide power during a power outage due to regulations that require shutting off power for safely reasons to protect line workers. However, an off-grid system can supply electricity when there is a grid power outage.

• Portable: An off-grid solar system doesn't have to be permanently attached to a structure and can be used in an apartment.

• No Government Involvement: No fees. no taxes, no regulations, and no inspections. Permits are not required for most things, but depending on what you want to do a permit might be required.

• UL Listed Not Required: In the United States, for a grid-tied system, you probably need UL listed products but for off-grid you don't but it is always nice to have.

• Not easy to use: In general an off-grid system is not going to as easy to use as a grid-tied system; although, it is possible to design a system that is as easy to use as grid-tied, most people probably won't design such a system. This is partly because of the government has regulations prohibiting easy DIY grid-tied systems. In other word, the government won't allow an easy to use DIY grid-tied system and instead only allows a complex grid-tied system that requires permits and often requires professional solar installer or if you DIY requires a lot of work dealing with the government and utilities.

• Privacy from electricity usage monitoring: With an off-grid solar system the government can't invade your privacy and monitor you electricity usage.

Grid-tied solar system

Generally a grid-tied solar system is installed by a solar system company but it is possible to DIY but that won't be covered here. 

In addition, there is a DIY grid-tied solar system you can buy that plugs into your houses outlet. I will mention these systems but won't cover how to install these DIY grid-tied solar system. Many of these DIY grid-tied solar system might not be legal but some might be legal.

• Expensive solar system cost: Typically installed by professionals and cost $10,000 and up depending on how much power you want. 

• Expensive maintenance and repair cost: You typically can't do maintenance and repairs yourself.

• Net metering: Net metering means selling solar electricity back to the grid. If you are grid-tied some power companies allow net metering. The amount you get from net metering varies with each power company.

• Permit Required: Typically a permit is required to connect a grid-tied solar system to the houses grid electricity. However, there are DIY grid-tied solar systems that might or might not be legal and might or might not require a permit.

• Power Outages: Often a grid-tied solar system can't provide power during a power outage due to government regulations that require shutting off power for safely reasons to protect line workers.

• Not portable: A grid-tied system has to be connected to the grid electricity on a house so generally isn't portable.

• Government Involvement: Government involvement might include taxes, fees, regulations, inspections, and permits.

• UL Listed Required: In the United States, for a grid-tied system, you generally need UL listed products which can be more expensive but for off-grid it is up to you what you want to buy and use.

• Easy to use: When the solar system is grid-tied to your houses electrical system it is generally easy to use and you use your electricity just like your always have.

• No privacy from electricity usage monitoring: With an grid-tied solar system the government could invade your privacy and monitor you electricity usage.

Reference: 

Solar net metering dirty secrets! A MUST WATCH before you buy your system. #767

The $94 Million Smart Meter Surveillance Scheme Exposed

What does off-grid mean?

Off-grid means your solar system is not connected to the grid but it does not mean your house is off-grid. In fact, in most cities it is not even legal to have your house off-grid so in most situations your house will have to be on-grid. 

Even though your house is on-grid you don't have to use any grid electricity; however, most cities require paying a monthly minimum grid-connection fee even if you don't use any electricity.

Since in most cases you have no choice but being connected to the grid and paying a minimum fee you might as well just consider the grid as backup power for your solar system. Even if you don't want to pay for a grid connection it is good to have unlimited backup power available just in case you need it.

Government Solar Subsidies

As of 2024. Both a professional installed grid-tied solar system and a DIY off-grid solar system can qualify for government solar subsidies such as the Federal Solar Tax Credit. As of 2025, the Federal Solar subsidies might be discontinued but you can check if state or other local subsidies are available in your area.

However, most if not all of the government solar subsidies available are tax credits, not cash payments so not everyone qualifies. You might not earn enough to get the subsidy, or you might earn too much to get the subsidy.

Note that sometimes you might see advertisements for free solar from the government for low income but as far as I know there is no such thing as free solar.

I'm not going to cover government solar subsidies, but you could ask a CPA or tax professional if you are interested in getting them.

When I create my DIY solar system I'm not even going to try to get any government solar subsidies, so I will pay the full cost of my solar system out of my own pocket. The reason is because I don't want to bother with the complexity of tax forms to try to figure out if I can get the tax subsidy from the government. However, for those that want to do the extra work you might be able to save extra if you qualify for a tax subsidy.

Solar Tariffs

If you want to know why solar cost so much you can thank the USA government.

Often solar systems in other counties will be significantly cheaper than in the United States because of tariffs.

The tariffs are determined by the government and can change anytime. However, once a tariff is put in place they tend to not want to remove them because the government likes taxes. For example, Trump put tariffs on China and Biden kept them and even increased some of them.

Unlike sales taxes tariffs could be any amount, even over 100%, because a tariff is a hidden tax.

Tariffs are a hidden tax because they don't show the tax on the sales receipt like the sales tax. 

Tariffs are going to increase the cost of solar regardless of where it is manufactured. For example, a tariff on China will also increase the cost of USA manufactured products because the USA manufacturers will be able to increase their prices because of less competition from China.

In the past the federal government has had a solar tax credit that you can get on your taxes if you qualify and apply for it but they also add tariffs to make solar more expensive. This is how the government makes it look like they are saving you money when they are really making it more expensive for you. You would save more money with no solar tax credit and no tariff. In other words, if the government allowed a free market and did nothing you would save more money.

Why SOLAR Panels Cost 3X MORE in USA vs Europe - The Hidden Truth About Solar Pricing 2025 

Cash vs Debt vs Lease Purchase 

Lease is a type of rental agreement where you don't own the solar panels placed on your house. I would not even consider using a lease to get solar.

If you get a loan to buy solar it will cost more because of the interest you have to pay so I would not use a loan myself.

In addition, if you get lease or load to get solar and want to sell your house in the future it can reduce the value of your house because not everyone will want to pay or be able to pay for your solar lease or loan.

When I buy a solar system I will pay cash to save money.

Ground vs Roof Installation

I would not want to install solar panels on the roof.

• Solar panel maintenance: It is safer and easier to clean, repair, and maintain your solar panels if they are installed on the ground.

• Roof repair: If a roof needs to be replaced or repaired it might cost more if solar panels are on the roof.

• Homeowners insurance: If you install solar panels on your roof your homeowners insurance company might drop you. Check with your homeowners insurance company before installing solar on your roof and find out if they allow solar panels on your roof or if your rates will go up.

How to Connect Solar Electricity to House

There are several ways you can connect electricity from your solar system to your house. 

1. Off-grid using extension cords. This is the easiest way to use solar in an apartment.

2. Off-grid using a transfer switch to breaker box. How I Cut My Electric Bill Using a Transfer Switch and Power Station! 

3. Off-grid  using an interlock kit to breaker box.

4. Grid-tie to outlets: This requires a grid-tie inverter that plugs into an outlet. See inverter section for more info.

5. Grid-tie direct to breaker box: This requires government and power company approved equipment to connect to the breaker box and requires a permit. This method is generally not a DIY project; although, it is possible and legal to DIY on your own house in some states but you will still need a permit. It probably is not legal to DIY on someone else's house unless you are a licensed electrician. Method five will not be covered by this document.

Electricity Rates by State

California has the highest electricity rates in the country at an average of $0.29 per KWh and Washington has the lowest at an average of $0.11 per KWh. Keep in mind these are the average rates in the state so the rate in different cities can be higher or lower.

Reference: https://www.chooseenergy.com/electricity-rates-by-state/

Electricity Rates California

Highest Rates

California has the highest electricity rates in the United States.

In the past there was only one rate for electricity; however, today there are many different rate plans similar to cell phone company's that have many rate plans. 

As of 2023, in California, I'm on the PG&E tiered plan and I pay a minimum of $0.47 per KWh so my rate is higher than the average rate in California of $0.29. My $0.47 rate increases on higher tiers. There are currently 3 tiers; although, they don't call the last tier a tier so they say there are 2 tiers. 

Some cities such as Las Vegas in other states have rates about $0.12 per KWh so I am paying about 5 times more than the lowest price rates in other states.

California government policy

In the past electricity generation was centralized and produced by power companies. Now the government wants electricity decentralized and produced by the individual using solar panels. 

In my optional, the reason electricity rates in California are higher than other states is government policy. The California policy is to price grid electricity rates high making it too expensive to use for many so those that can't afford it are forced to either go without or buy solar panels to save money.

California has the highest housing prices in the nation which means fewer people can buy a house so they are forced to rent. Renters can't easily install solar panels so they are forced to pay the high price of electricity.

High electricity rates will hurt poor people but won't have any effect on rich people because they won't have any problem paying the higher prices or buying solar panels to save money.

Reference: Why California Electricity Is So Expensive | Susan Shelley 

Grid Connection Fee

High electricity rates encourages people to stop using electricity from the grid which would eventually put power companies out of business. 

Unfortunately, the law in most cities requires being connected to the grid and paying a Grid Connection Fee each month whether you use any electricity or not.

As of 2024, California’s PG&E’s minimum fee is $11.90 per month according to the company’s website.

On 5/9/2024, the Public Utility Commission (PUC) passed a new income based flat fee plan that will replace this old $11.90 per month grid connection minimum fee with a higher fee.

Reference: https://www.pge.com/en_US/residential/rate-plans/how-rates-work/rate-changes/minimum-bill-charges/minimum-bill-charges.page 

Income Based Fixed Rate Electricity Plan

On 5/9/2024, the Public Utility Commission (PUC) in California passed an income based flat fee of about $24.99 per month with possible discounts if you can prove you are low income. There will be a small decrease in the rate per KWH but they will just raise it in the future so it won't really help.

In addition, just the fact that you have to apply for the low income discount means it is not easy and many will probably not want to bother with it. Also if your income changes updating this information with the utility will not be easy.

It is important to note that this is not really a flat fee, it is a flat fee plus the rate per KWH. This means that even if you use zero electricity you have to still pay the flat fee. PG&E actually had a fixed flat fee for everyone before this new plan that was $11.90 a month even if you use zero electricity.

This plan is supposedly to help the poor; however, I think the real reason for this income based pricing scheme is that many solar users are essentially zeroing out their electric bills and the power companies want to increase the minimum fees so that those using no electricity with solar will have to pay a higher minimum fee for being connected to the grid.

"Flat Fee" Utility Scam!

Payback time

The best way to determine if solar panels are worth it is to calculate your payback time.

In states with high electricity cost like California the payback time is probably worth it but in states with low rates like Las Vegas it probably is not worth it to save money. However, even in California you have to be careful to make sure your payback period is as short as possible to make it worth it.

Below are reasons someone might want solar.

1. Saving money.

2. As a backup to the grid electricity going down.

3. No grid electricity available because you live outside of the city or in an RV.

4. You don't want to use grid electricity to save the environment.

If your reason is anything other than, number one, saving money then the cost might not be your primary concern. As an example, if you want solar for a backup if the grid goes down you might not care if you save money.

To save money, the cost of the solar system must be paid back in savings from your electricity bill. Everything after you paid back the cost of your solar system will be profit.

The faster the payback time, the more money you will save.

In my opinion, you should have a payback period of 3 years or less.

Below are factors that effect the payback time.

1. Solar System Cost: The lower cost of the solar system, the faster payback.

2. Cost of Electricity: The higher the cost of electricity, the faster the payback.

3. Energy Usage: The more electricity you use, the faster the payback.

In addition, you need to get sun to save money because if you get no sun you can't produce electricity.

Government subsidies and the ability of getting paid for selling electricity back to the grid can also reduce your costs.

Note that most power companies have a minimum monthly fee that you have to pay even if you use zero electricity so in most situations you won't be able to have a zero power bill. If you live in an area that allows selling electricity back to the power company it might be possible to zero out your electric bill but that requires a grid-tied solar system which is not covered in this document.

The Real Truth about Solar Savings (and Why Solar Is Still Worth It!)

Payback Calculator

You can use the payback calculator below to calculate your payback time.

Solar ROI Calculator 

You need the following information to determine your payback period.

• Solar System Cost: This is how much you paid for your solar system.

• Cost of Electricity: You can find this information from your electricity bill.

• Yearly Energy Usage: You can find this information from your electricity bill usually reported in kWh.

Energy used by appliances

You can calculate your own energy use using the website below.

Estimating Appliance and Home Electronic Energy Use 

My actual rates in California are around $0.40 to $0.51 kWh in 2025 instead of $0.2704 kWh the website is using.

• Aquarium: 24W 1 hours/day 365 days/year 8.8 kWh $2.37

• Cable box: 140W 1 hours/day 365 days/year 51.1 kWh $13.82

• Ceiling fan: 35W 1 hours/day 365 days/year 12.8 kWh $3.45

• Clothes dryer: 2790W 1 hours/day 365 days/year 1,018.4 kWh $275.36

• Clothes washer: 255W 1 hours/day 365 days/year 93.1 kWh $25.17

• Coffee maker: 1000W 1 hours/day 365 days/year 365.0 kWh $98.70

• Compactors: 400W 1 hours/day 365 days/year 146.4 kWh $39.58

• Computer desktop: 75W 1 hours/day 365 days/year 27.4 kWh $7.40

• Computer laptop: 25W 1 hours/day 365 days/year 9.1 kWh $2.47

• Deep fryer: 1000W 1 hours/day 365 days/year 365.0 kWh $98.70

• Desktop computer monitor: 42W 1 hours/day 365 days/year 5.3 kWh $4.15

• Dishwasher: 330W 1 hours/day 365 days/year 120.5 kWh $32.57

• Electric blanket: 400W 1 hours/day 365 days/year 146.0 kWh $39.48

• Electronic air cleaner: 50W 1 hours/day 365 days/year 18.3 kWh $4.93

• Furnace fan: 295W 1 hours/day 365 days/year 107.7 kWh $29.12

• Garage door opener: 400W 1 hours/day 365 days/year 146.0 kWh $39.48

• Hair dryer: 710W 1 hours/day 365 days/year 259.1 kWh $70.07

• Humidifier: 11W 1 hours/day 365 days/year 4.0 kWh $1.09

• Iron: 1100W 1 hours/day 365 days/year 401.5 kWh $108.57

• Lawn sprinkler: 11W 1 hours/day 365 days/year 4.0 kWh $1.09

• Microwave oven: 1500W 1 hours/day 365 days/year 547.5 kWh $148.04

• Pool pump: 1000W 1 hours/day 365 days/year 365.0 kWh $98.70

• Portable spa: 4350W 1 hours/day 365 days/year 1,587.8 kWh $429.33

• Printer inkjet: 13W 1 hours/day 365 days/year 4.7 kWh $1.28

• Printer laser: 250W 1 hours/day 365 days/year 91.3 kWh $24.67

• Printer multi function: 18W 1 hours/day 365 days/year 6.6 kWh $1.78

• Receiver: 28W 1 hours/day 365 days/year 10.2 kWh $2.76

• Rechargeable power tool: 13W 1 hours/day 365 days/year 4.7 kWh $1.28

• Refrigerator: 225W 1 hours/day 365 days/year 82.1 kWh $22.21

• Router/DSL/Cable modem: 6W 1 hours/day 365 days/year 2.2 kWh $0.59

• Slow cooker: 200W 1 hours/day 365 days/year 73.0 kWh $19.74

• Space heater: 1320W 1 hours/day 365 days/year 481.8 kWh $130.28

• Stereo systems: 33W 1 hours/day 365 days/year 12.0 kWh $3.26

• Television LCD: 150W 1 hours/day 365 days/year 54.8 kWh $14.80

• Television set top box: 20W 1 hours/day 365 days/year 7.3 kWh $1.97

• Toaster: 1100W 1 hours/day 365 days/year 401.5 kWh $108.57

• Toaster oven: 1051W 1 hours/day 365 days/year 383.6 kWh $103.73

• Vacuum: 542W 1 hours/day 365 days/year 197.8 kWh $53.49

• Video game system: 36W 1 hours/day 365 days/year 13.5 kWh $3.65

• Water heater: 4500W 1 hours/day 365 days/year 1,642.5 kWh $444.13

• Well pump: 725W 1 hours/day 365 days/year 264.6 kWh $71.55

My energy use

My electricity rates

Looking at my last two PG&E bills in 2025 I used 196 and 256 kWh per month and my rate was 0.39 to 0.51 per kWh and my charge for electricity was $78 to $106.

To keep it simple I will round the amount I pay for electricity to about $100 a month or $1200 a year. 

Conserving electricity

I have both gas and electric appliance. Below are ways I try to save on gas or electricity.

• I don't use the central gas heater.

• I don't use an electric space heater.

• I do use an electric heating pad.

• I don't use heat dry mode on the dishwasher.

My current electricity bill is the result of me trying to conserve as much electricity as I can. For example, to conserve electricity I don't use the heat dry mode on the dishwasher which means I have to dry my dishes outside of the dish washer taking extra time and effort and reducing my qualify of life. My bill would be higher if I was not trying to conserve electricity.

The amount I save depends on how much electricity I would use if not conserving electricity, not how much I'm actually using now while conserving electricity.

If you wanted to keep track of exactly how much you actually save you would need to keep track of how many kWh of solar electricity you use and most people don't keep track of that.

Electric appliances 

Most estimated cost are from the website calculator below.

Estimating Appliance and Home Electronic Energy Use 

For the Refrigerator I used 411 kwh per year from the manufacturer and used the electricity rate of 0.40. 

I am not including the cost of gas for my gas furnace, water heater, and oven.

To keep it simple I only included estimated cost for the things I think use the most energy. 

Total: $980.32

• Aquarium: 

• Cable box: 

• Ceiling fan: 

• Clothes dryer: 2790W 1 hours/day 365 days/year 1,018.4 kWh $275.36

• Clothes washer: 255W 1 hours/day 365 days/year 93.1 kWh $25.17

• Coffee maker: 

• Compactors: 

• Computer desktop: 

• Computer laptop: 

• Deep fryer: 

• Desktop computer monitor: 

• Dishwasher: 330W 1 hours/day 365 days/year 120.5 kWh $32.57

• Electric blanket: 

• Electronic air cleaner: 

• Furnace fan: 

• Garage door opener: 

• Hair dryer: 

• Humidifier: 

• Iron: 

• Lawn sprinkler: 

• Microwave oven: 1500W 1 hours/day 365 days/year 273.8 kWh $74.02

• Pool pump: 

• Portable spa: 

• Printer inkjet: 

• Printer laser: 

• Printer multi function: 

• Receiver: 

• Rechargeable power tool: 

• Refrigerator: 411 kwh $164.40

• Router/DSL/Cable modem: 

• Slow cooker: 

• Space heater: 

• Stereo systems: 

• Television LCD: 150W 24 hours/day 365 days/year 1,314.0 kWh $355.31

• Television set top box: 

• Toaster: 

• Toaster oven: 

• Vacuum: 542W 1 hours/day 365 days/year 197.8 kWh $53.49

• Video game system: 

• Water heater: 

• Well pump: 

Low Energy Appliances

A lot of appliances were designed when energy was plentiful because the government provided cheap energy on the grid; however, it is expensive for you to provide off-grid solar energy.

In this section we look at if you should buy new low energy appliances as a replacement for high energy appliances to run off solar.

What loads are most practical to run on solar?

You can run any load on solar but some loads require so much power that it might not be economically practical.

Solar is best suited to running loads that are either low wattage (i.e. 7 watt LED lightbulb for 12 hours a day) for long periods of time or high wattage (i.e. 1000 watt toaster for 5 minutes) for short periods of time. 

Running high wattage loads for long periods of time is generally not practical (i.e. 1500 watt space heater for 24 hours a day) because the solar system required would be so expensive that it is not cost effective.

• Small Appliances: Small appliances such as a instant pot, toaster, toaster oven, coffee maker, blender, mixer, can all be run on solar for short periods of up to 10 minutes several times a day without a problem.

• Microwave: A standard microwave could be run on solar; however, an inverter microwave is better because they use less power. A standard microwave uses full power in an on and off manner when set to a lower power setting. An inverter microwave when set to a lower power setting actually uses less power. A standard microwave that is rated at 1000 watts would use about 40% more power or about 1400 watts total. So although you can run a standard microwave it will require more power than an inverter microwave.

• Heating: Heat pumps can heat or cool a house. Small heat pumps are best for solar and some can run on 300 to 400 watts. Resistive space heaters use from 1000 to 1500 watts so they are generally not practical to run on solar because of how much power they use.

• Air Conditioning: An air conditioner can be run off of solar but they do require at least around 400 watts for a small window air conditioner. You can also run them off of batteries but that would require a lot of battery power so it generally is not a good idea.

• Clothes Dryer: I would not try to run a 240V clothes dryer because of the added expense of a 240V solar system. I would buy a 120 volt electric clothes dryer that is resistive or uses a heat pump. A heat pump dryer would be the best because it uses less energy. I would estimate about 1500 watts for a 120V resistive heat clothes dryer and about 500 watts for a 120V heat pump clothes dryer.

• Water Heater: You can use a 120 volt resistive water heater or 120 volt heat pump water heater.

• Oven: A 220 Volt Oven requires too much power to be practical to run off of solar. There are 120 Volt heat pump ovens as an alternatives.

• Stove Top: Use a 120V AC induction cook top.

• Refrigerator: A standard refrigerator might use 6.8 amps or  816 watts but a mini refrigerator might only use 75 watts. In addition, a DC refrigerator won't require running an inverter all day so might save energy that way too.

• 220 Volt EV charger: I need to do more research on this subject so I will add more here later.

Price per Watt vs Cost per kWh

There are two main ways to calculate the cost of a solar system:

• Price per watt (PPW): Compares multiple solar system. Total system cost / Total Watts.

• Cost per kilowatt-hour (cents/kWh): Compares the cost of solar versus grid energy. Total cost / Lifetime kWh produced.

Cost per kWh

The estimate for cost per kWh would be the entire cost of the system including maintenance cost for the life of the system divided by the kWh produced during the life of the system as shown below.

Total cost of solar system / Total kWh produced

Batteries are optional depending on if you want electricity at night and add to the cost of the solar system. If you have fewer batteries you will get a lower cost per kWh but you might not have enough electricity at night when you want it. In other words, batteries add to the total cost of the solar system and will increase your cost per kWh .

I'm using a lifespan of 30 years for the solar system.

100 Watt Solar System Cost per kWh 

A 100-watt panel for an average of four hours of sunlight produces about 0.4 kWh per day. 

1000W 12V Solar System Total $314.95 + Wiring = about $400.

0.4 kWh per day x 365 days in year = 146 kWh per year x 30 year lifespan = 4,380 kWh in lifespan.

$400 total cost / 4,380 kWh per lifespan = 0.10 per kWh .

200 Watt Solar System Cost per kWh 

A 200-watt panel for an average of four hours of sunlight produces about 0.8 kWh per day. 

1000W 12V Solar System Total $314.95 + Wiring + $100 extra solar panel = about $500.

0.8 kWh per day x 365 days in year = 293 kWh  per year x 30 year lifespan = 8,760 kWh in lifespan.

$500 total cost / 8,760 kWh per lifespan = 0.05 per kWh .

Solar System

A solar system typically has four components consisting of a solar panel, charge controller, battery, and inverter. 

However, components can be combined in one device. The disadvantage of combining components is it is less flexible and might cost more to repair.

One combination is a solar generator that combines a charge controller, battery, and inverter in one device so you only need to add a solar panel.

Another combination is called a hybrid system with the charge controller and inverter in one device that you can add solar panels and batteries to.

1. Solar Panels: Produces electricity from the sun. If you just want battery backup incase of power failure then you would not need solar panels.

2. Charge controller: A charge controller is a battery charger that uses electricity from solar panels to charge the battery. If you just want battery backup incase of power failure then you would not need a charge controller.

3. Battery: The battery stores power from generated from the solar panels for use when there is no sun. Both off-grid and grid-tied solar systems can be designed without a battery. Most off-grid systems use a battery; however, many grid-tied systems don't use a battery and rely on the grid when there is no sun.

4. Inverter: Converts power from your battery to 110 or 120V AC for use with household appliances. You only need an inverter if you want 120V AC power. 

Solar Panels

What type of solar panel is best?

There are three basic types of solar panels, monocrystalline, polycrystalline, and thin film.

Polycrystalline is about 18% efficient compared to monocrystalline 22% efficient.

Polycrystalline is usually cheaper than monocrystalline because they are easier to manufacture. 

Monocrystalline is the most popular solar panel but if you get a good price on polycrystalline solar panels there is nothing wrong with using them.

Thin-film are flexible, not as rugged, and generally cost more so they are usually best for portable solar panels or for mounting on cars.

What is a shade tolerant solar panel?

Another type of solar panel is the shade tolerant solar panel available in either rigid or flexible form. It is designed to provide more power when some cells are shaded; however, these panels are usually more expensive. If your panels are never in the shade you don't need this feature.

Are mono-facial & bifacial solar panels best?

Mono-facial solar panels only produce power from the front side.

Bifacial solar panels are designed to produce the rated power from the front side and can produce extra power from the backside.

Bifacial solar panels often cost more than mono-facial solar panels but they might be the same price or even lower.

Bifacial solar panels higher off the ground on a ground mount will get more power from the backside than a roof mount close to the roof. You can still mount bifacial solar panels close to a roof but you likely won't get any extra power from the backside.

If everything else is equal including price I would get the bifacial solar panels.

In the future it could be that most solar panels are bifacial solar panels because they do not seem to really cost more to manufacture.

What is a 12 volt solar panel?

A 12 volt solar panel does not have an output voltage of 12 volts but instead produces a voltage that is optimum for charging a 12 volt battery so generally around 16 to 20 volts.

In addition, a 12 volt battery is not really 12 volts but 12.6 volts. A battery needs about 14-15 volts to charge.

How many watts of solar panels do you need to charge a 12V 100 Ah in a day?

The short answer is you will likely need 300 to 400 watts of solar panels to charge a 12V 100 Ah in a day. This is important because 12V 100 Ah LifePo4 batteries are a common size uses for solar systems.

Below is from google AI.

AI Overview

To charge a 100Ah 12V battery in a day, you will need 200W to 400W of solar panels, with a 300W panel being a good recommendation for balancing performance and cloudy days. The exact number depends on factors like sunlight conditions, panel efficiency, and battery type. 

Panel size recommendations

• Minimum: A 200W panel is the minimum needed under ideal, sunny conditions. 

• Recommended: A 300W panel is recommended to provide a buffer for less-than-perfect weather and ensure reliable charging. 

• Optimal: A 400W panel will ensure the battery is fully charged even on days with partial sun. 

Factors to consider

• Peak Sun Hours: The number of hours of intense sunlight in your location. For a 300W panel, you'd need about 5 peak sun hours to fully charge the battery, notes Future Heat. 

• System Efficiency: A more efficient charge controller, like an MPPT controller, can increase daily power generation by up to 30% compared to a PWM controller, says JINGSUN. 

• Battery Type: Lead-acid batteries may require a larger charging redundancy to avoid overcharging damage, while lithium-ion batteries can handle higher charging currents from more powerful panels, notes JINGSUN. 

• Shadowing: Even partial shading on the solar panels can drastically reduce their efficiency, making it crucial to place them in an unobstructed area, notes Future Heat. 

• Battery Depth of Discharge: How much power you use overnight will affect how much energy needs to be replaced. A 100Ah battery needs about 1200 watt-hours (100Ah x 12V) to be fully charged. 

How many amps does a 100 watt panel produce?

Calculate the current in amps by dividing power in watts by the voltage in volts. 

For a 12V solar panel, the voltage will be about 18 volts. Since watts equals volts times amps, amperage will be equal to 5.5 amps (100 watts divided by 18 volts).

Example: A 12 volt 100 watt solar panel will produce about 5.5 amps at 18 volts.

Adding solar panels in series or parallel

You can add one or more solar panels by connecting them in series or parallel.

• Series: Add the voltage and take the lowest amps. Lower amps allow using thinner wire to connect panels to the charge controller.

• Parallel: Add the amps and take the lowest voltage. This allows keeping the voltage lower so you don't go over the voltage max input for your charge controller.

Generally, it is better to have solar panels that match the voltage and amps but that is not required. 

As an example, if you have two 12V solar panels connected in series but one is 10A and the other is 5A than the output will be 5A so you will lose 5A of potential output power because they are mismatched.

Should I connect my solar panels in series or parallel?

Choosing to connect solar panels in series or parallel often depends on the type charge controller you are using. With PWM, parallel is often required or else the voltage would be too high. With MPPT, series is often better because the higher voltage keeps the amps low allowing you to use thinner wire.

You can also use a combination where some panels are in series and some parallel.

What gauge wire should I use to connect the solar panel to the charge controller?

10AWG is a typical value to use for most applications.

Why does a 12V solar panel have a voltage of around 17V?

A solar panel that is called 12V produces around 17V. The reason it is called a 12V solar panel is because it is designed to be used with 12V batteries. The 12V solar panel has to produce a higher voltage that will be reduced to around 12V by a charge controller for charging a battery.

What is Open Circuit Voltage (Voc)?

The is the maximum voltage from the solar panels with no load.

Does the Solar Panel Voc have to be less than the Maximum PV Input Voltage of the charge controller?

You must make sure your solar panels are connected properly for your charge controller to prevent damaging your charge controller. 

For a 12V solar panel a typical Voltage Open Circuit (VOC) is 21.6V which is the maximum voltage produced by the solar panel. You have to make sure the VOC is less than the Maximum PV Input Voltage of your charge controller. 

Often a PWM charge controller will have a Maximum PV Input Voltage of 23V so this would mean you have to connect multiple solar panels in parallel so you don't go over the max voltage limit. An MPPT inverter will often have a Maximum PV Input Voltage of 50V or more which will give you more flexibility in how you connect multiple solar panels.

Charge controller

Should you get a PWM or MPPT charge controller?

A MPPT charge controller is 30% more efficient than PWM but costs more. 

The PWM charge controller typical has a max solar voltage input of 40 volts. If you connect multiple solar panels in series the voltage can become too high; therefore, often parallel is used to keep the voltage low but this increases the amps which requires thicker and more expensive wire.

The MPPT charge controller typically has a higher max solar voltage input than PWM. This allows using series to connect your solar panels keeping the amps low amps and using a thinner and cheaper wire. However, this increases the voltage which is a dangerous shock hazard.

What charge controller specifications do you need?

• Battery Type: Select a charge controller that is capable of charging the type of battery you have. For example, for a LifePO4 battery get a charge controller that can charge a LifePO4 battery. 

• Battery Voltage: Select a charge controller that is rated to charge the voltage of your battery. Often solar charge controllers are rated for multiple voltages. For example, it might be rated for 12V and 24V.

• Maximum PV Input Voltage: Select a charge controller with a max solar input voltage that is greater than the solar voltage feeding into the controller. The total voltage from your solar panels depends on your solar panel voltage and how many panels are connected in series.

• Maximum PV Input Power: Select a charge controller with a charge current that is greater than the total amps from your solar panels.

How many solar panels can I connect to my charge controller?

As you add solar panels you must make sure that the max solar panel (PV) voltage and amps remain below your charge controllers Max PV Input Voltage and Max PV Charge Current that it can handle.

Never go over the charge controllers Max PV Input Voltage or else it will burn up your controller.

Sometimes you can go over your charge controllers Max PV Charge Current to some degree which is called over paneling but there is a limit.

Tip: The MPPT charge controller accepts higher voltage than PWM so you can connect solar panels in series which increases the voltage and keeps the amps the same allowing using thinner wires.

Battery

What type of battery chemistry is best?

• In 2024, Solid State batteries are starting to become available for DIY off-grid solar systems; however, it is not yet clear if they will be better and cheaper than LifePO4 batteries.

• In 2024, Sodium Ion batteries are starting to become available for DIY off-grid solar systems; however, it is not yet clear if they will be better and cheaper than LifePO4 batteries. Some say the discharge curve is not anywhere near as good as LifePo4 batteries so keep that in mind when evaluating Sodium Ion batteries.

• From 2023 to 2024, LifePO4 (lithium phosphate) batteries have been the most popular type of battery for DIY off-grid solar systems. The advantages of LifePO4 batteries are that they are maintenance free, have a longer lifespan than lead acid and lithium ion batteries, and are less likely to catch fire than lithium ion batteries. The disadvantages of LifePO4 batteries are that they are heavier and bulkier than lithium ion batteries.

What physical size of battery is best?

I prefer Group 24 because it is the smallest but I would generally buy whatever is cheapest as long as the battery size fits my space.

• Group 24: 10.5” L x 6.1875” W x 8.875” H.

• Group 27: 11" L x 7.3" W x 9.3" H.

• Group 31: 12.8" L x 6.8" W x 9.3" H.

What terminal or lug size does my battery have?

The most common terminal or lug size for a LifePo4 100 Ah battery is M8 (8mm or 5/16 inch); however, always check the specifications of your battery model to confirm the correct lug size. 

For an M8 terminal, the M stands for metric and the number after, in this case 8, stands for the 8 mm diameter of the bolt.

• 1/4” (M6)

• 5/16" (M8)

• 3/8" (M10)

What battery voltage should I use?

The most common voltages for solar system batteries are 12V, 24V, 36V, and 48V. Solar system batteries over 48V are currently not common but might be in the future.

One advantage of using a 12V battery is you can run 12V appliances such as a car tire inflator directly from the battery without a DC to DC down converter. With a 24V, 36V, or 48V battery you would need a DC to DC down converter to get 12V. 

A disadvantage of higher voltage batteries is the higher risk of shock. Another disadvantage is higher voltage batteries might cost more.

The main advantage of higher voltage batteries is you can use thinner wires due to lower currents. 

The example below shows that at the same 1800W, as the voltage increases from 12V to 48V, the amps decreases from 150A to 37.5A. You get the same power but at lower amps which allows using a thinner wire. 

• Amps (I) = Watts / Voltage

• 150A = 1800W / 12V

• 75A = 1800W / 24V

• 37.5A = 1800W / 48V

I recommend the following voltages as a general guide; however, you can always use a higher voltage if you prefer. As the power (watts) of your system goes up you almost have to switch to 48V because if you don't your wires have to be very big to handle the amps.

• A 12V battery is recommended for up to a 2000W inverter.

• A 24V battery is recommended for up to a 4000W inverter.

• A 48V battery is recommended for up to a 8000W inverter.

How long will the battery run a load?

The more Ah a battery has determines how long it can run a load before needing to be recharged. In an EV car you could call this range or how many miles a car can drive before needing to be recharged.

The more Amp Hours (Ah) a battery has, the more power it has which is expressed in What Hours (Wh).

Often the battery only gives you the Ah but you can convert Ah to Wh.

To determine how much power your battery can produce you must know the Wh of your battery. 

The example below shows how to convert Ah to Wh. In the example, I calculate a 12.8V 100 Ah battery has 1280 Wh of power.

• Battery: 12.8V 100 Ah

• Formula Convert Ah to Wh: Wh = Ah  × V.

• Answer: 1280 Wh = 100 Ah * 12.8V

As an example, if you are running a 1000W space heater for 1 hour it will use 1000Wh of power so with a 12V 100 Ah battery you would have a maximum of 1280 Wh so that could be enough to run a 1000W space heater for 1 hour or more. Keep in mind that if you are using an inverter to convert 12V DC to 120V AC that the inverter itself will use some power too while it is running the space heater for 1 hour.

What is the max watts or Max Discharge Current a battery can provide?

The max watts a battery can provide is determined by the batteries Max Discharge Current.

For example, if the battery has a Max Discharge Current of 100A then the max power you can get from a 12V battery is 12.8V x 100A = 1280 watts.

Smaller batteries generally have a smaller maximum discharge current.

You can connect two batteries in parallel to double the Max Discharge Current. As an example, if you have two 12V batteries that each have a Max Discharge Current of 100A or 1,280 watts and you connect the batteries in parallel then you would have a Max Discharge Current of 200A or 2,560 watts available from the two batteries connected in parallel.

How to size the battery to the inverter?

• Lead Acid: 100 Ah x 0.2C = 20A discharge rate

• LifePo4: 100 Ah x 1C = 100A discharge rate

Inverter: 1000W / 12V = 83A

The inverter amps should be lower than the battery amps discharge rate.

1000W Inverter: How Many Batteries You Really Need 

Adding Batteries in Series or Parallel

You can add one or more battery together and connect them in series or parallel.

• Series: Add the voltage and take the lowest amps. Lower amps allow using thinner wire to connect panels to the charge controller.

• Parallel: Add the amps and take the lowest voltage. This allows keeping the voltage lower so you don't go over the voltage max voltage input for your charge controller.

How long can a space heater run on battery?

A space heater typically uses 500, 750, 1000, or 1500 watts.

A 1000 watt space heater running for 1 hour will use 1000 Wh.

A 12 volt 100 Ah battery can produce 1280 Wh of power. 

After running the space heater for 1 hour, 1000 Wh will be used from the battery leaving 280 Wh minus whatever the inverter itself used up during the hour.  The inverter uses some power when it runs.

• 1280 Wh - 1000 Wh = 280Wh

• 280 - Inverter Wh used

This was just an example, generally space heaters are not used on battery power because of the high cost of batteries which currently are about $200 for a 12V 100 Ah battery.

What size battery do you need to run an entire house?

Typically to run an entire house you would need 5 to 10 kWh.

Solar battery cost: Why they're not always worth it 

Inverter

Why do I need an inverter?

An inverter is needed because most home appliances run on 120V AC and most solar systems run on 12V, 24V, 36V, or 48V DC. If you wanted to buy all new appliances that run on whatever DC voltage you are using then you would not need an inverter.

How much power in watts of an inverter do I need?

You need an inverter that provides enough power in watts for the appliance you want to run. 

Some appliances such as a refrigerator also have a surge rating in watts so your inverter will also have to have enough surge rating in watts for some appliances.

Do I need a modified or pure sign wave inverter?

A pure sign wave inverter is more expensive than a modified sign wave inverter but should run all appliances just the same as from the grid.

A modified sine wave inverter is cheaper than a pure sign wave inverter but some appliances like refrigerators, microwaves, electric pressure cookers, and compressors might not run as efficiently or might not run at all. 

Unless you know a modified sine wave inverter will run everything you want to use you are better off getting a pure sign wave inverter.

What is the standby or idle consumption in watts of an Inverter?

An inverter is not 100% efficient so just having an inverter turned on will use some power in watts known as standby or idle consumption.

An inverters standby consumption is generally 10 to 50 watts per hour. To find the exact amount you will need to check the specifications of your inverter. In general smaller inverters have lower standby consumption.

Should the inverter be connected to the charge controller load output or directly to the battery?

The short answer is that in most cases you should connect the inverter directly to the battery.

The reason in some cases the inverter is connected to the charge controllers output is because some charge controllers have a low voltage shutoff to protect the battery from being drained too low. This is primarily to protect Lead acid and AGM batteries that should not be drained too low before recharging because it can damage the battery.  LifePo4 batteries are safter to drain all the way.

First of all some charge controllers don't have a load output so you have to connect the inverter directly to the battery.

Second some charge controllers are not designed to have the inverter connected to the load output.

If the charge controller is designed to allow connecting the inverter to its load output you have to make sure the inverter does not need more power then the charge controllers load output can provide.

Some charge controllers don't offer enough power from the load output so by connecting to the battery you can get more power from your inverter. 

Some inverters have a low voltage shutoff so this is another way to get that feature if you want to protect your battery from draining too much.

Best Way To Wire Inverter? Battery vs. Charge Controller 

What input voltage of inverter do I need?

You need an inverter that matches your battery voltage. As an example, if you are using a 12V battery you need a 12V inverter and if you are using a 24V battery you need a 24V inverter, etc.

Is 120 volt better than 110 volt?

Today 120 volts is the standard voltage for residential households in the United States but almost all household appliances and electronic equipment should run fine on 110 volts too. There might be some equipment that has problems running on 110 volts but that would not be very common so I would not generally worry about if the inverter output is 120 or 110 volts. That being said if you are worried about compatibility then I would get an inverter with 120 volts output because that exactly matches the standard voltage from a residential outlet in the United States.

How many watts does a 120V electric outlet in the United States use?

In the United States, 120V outlets are either 15 or 20 amps.

120V 15 amp outlets

The 120V 15A outlet is the most common electric outlet in the United States. 

120V 15 amp outlets use 14 AWG wire.

The 120V 15A outlet uses 1800 watts at 120 volts as shown by the V * A = W formula below. 

• V * A = W

• 120V * 15A = 1800W

This means that to run any home appliance such as a TV, Microwave, Toaster, or Space Heater one at a time you need an inverter that can supply at least 1800 watts continuously.

120V 20 amp outlets

There is also a 120V 20 amp outlet for use with large appliances like refrigerators and washing machines. 

120V 20 amp outlets use 12 AWG wire.

The 120V 20A outlet uses 2400 watts at 120 volts as shown by the V * A = W formula below. 

• V * A = W

• 120V * 20A = 2400W

Reference:

How Do 15A and 20A Outlets Differ? What You Should Know 

How many watts do I need to run the average appliance?

Note these are rough estimates for an average appliance so you will need to check how many watts your appliance uses to be sure what the correct value is for you appliance.

• Standard refrigerator: 300 to 800 watts.

• Mini refrigerator: 50-180 watts.

• Window air conditioner: 500 to 800 watts.

• 2 slice toaster: 700 to 1100 watts.

• Small Microwave: 500 to 800 watts.

What is the max wattage a space heater or hair dryer uses?

Both a space heater and hair dryer are designed to be used on a standard 1800 watt outlet. A space heater is only allowed 1500 watts max out of 1800 watts because it is used continuously. A hair dryer is allowed the full 1800 watts max out of 1800 watts since it is not intended to be used continuously.

How many watts do I need to run a full size refrigerator?

I have the full size refrigerator below and on the inside it says it is 6.5 amps so it uses 780 watts (6.5 amps x 120 volts).

Whirlpool 18.2 cu. ft. Top Freezer Refrigerator in White Model # WRT318FZDW 

According to Microsoft copilot below in bold.

A 6.5 amp refrigerator would typically require an inverter with a capacity of at least 1000 watts to function properly, as you need to factor in the startup surge when the compressor kicks on, which can be significantly higher than the running wattage; this translates to around 2-3 times the average running power. 

A 1000 watt inverter might run a 780W refrigerator but I would prefer 1500 watt or higher to be safe.

In the YouTube video below he runs a similar size refrigerator to mine with a 1500 watt inverter.

How long can a 12v 100ah LifePO4 Battery run a 20 year old Full Size Refrigerator? Let's find out! 

What terminal or lug size does my inverter have?

The typical terminal or lug size for inverters around 1000 watts and above is 3/8 inch; however, always check the specific specifications of your battery model to confirm the correct lug size.

What is a Low Frequency Inverter?

A Low Frequency Inverter has a larger transformer and are heavier, can handle larger surges, has a higher idle current, are more reliable., and cost more. Most inverters are not Low Frequency Inverters because they cost more.

What is an inverter pre-charge resistor?

An inverter pre-charge resistor is used to prevent sparks when connecting an inverter to the battery. A typical value is 25W 30 Ohms that you use in series with the positive battery cable of the inverter when connecting to the battery and then remove after a second when it is pre-charged and then connect the inverter cable directly to the battery.

What is a DIY grid-tie inverter?

A grid-tie inverter takes power from a solar panel and pumps it into your houses electrical system while your grid power is live.

This is what a typical professionally installed solar system does; however, those generally require a permit to install and approval of your power company.

However, they do sell grid-tie inverters that you can install yourself (DIY) without a permit. Below is one example, of a grid-tie inverter you can buy from Amazon.

Y&H 1000W Grid Tie Inverter Stackable MPPT Pure Sine Wave DC30-45V Solar Input AC90-140V Output for 24V 30V 36V PV Panel 

The grid-tie inverter takes DC input from your solar panel and outputs 120V AC that you plug directly into your houses live electrical outlet. It feeds power into your houses electrical system so the power it produces will be used before using any grid electricity. Warning never plug a normal inverters 120V AC output into your houses live electrical outlet.

Are DIY grid-tie inverters legal?

As of 2025, Utah has made grid tie inverters legal but check that the grid tie inverter you buy is approved for use in your area if you buy one.

This Solar Panel is ILLEGAL (except in Utah)

Plug-In DIY Solar Is FINALLY Legal! Save $$$ Instantly! (Utah Just Changed Everything) 

Anti-islanding

Most grid-tie inverters have a feature called anti-islanding. Anti-islanding shuts off your grid-tie inverter if your grid power does down so it does not feed power into the grid during a power outage to protect line workers from getting shocked.

Sync 120V 60 Hz Frequency to Grid

The grid-tie inverter syncs the 120V 60 Hz frequency to match your grid power.

Back Feeding

Back feeding the grid is generally not allowed by power companies. The power company might be able to detect if you are back feeding power and if so might disconnect your power if you don't stop it. Smart meters can inform the power company if you are back feeding but mechanical meters can't. If you can keep the power generated less than what you are using it won't back feed. Some grid-tie inverters have current sensors that can connect to your breaker panel to detect how much current you are using and limit the current generated so it won't back feed. If you can get this to work it would not back feed.

The video below shows a grid-tie inverter setup to monitor the total power being used by the house so it will never back feed power to the grid.

Reference: https://www.youtube.com/watch?v=stJV-i0CpNQ

Conclusion

The problem with using a grid-tie inverter is the power company might not allow it, might disconnect your power if they detect it, or might charge you a fee if you feed back electricity to the grid.

The bottom line is that there are no DIY grid-tie inverters that I know of that are approved by the government and power companies where you can use it with a guarantee you won't get in trouble. The advantage of off-grid is that you can't run into problems with the power company or government.

Although the grid-tie inverter concept is great if it works I am not going to risk trying it. If you try a DIY grid-tie inverter try it at your own risk.

If there was a power company and government approved DIY grid-tie inverter available at a reasonable price that I could use without a permit or asking permission of the power company or government I would try it. The power companies and government have made no effort to make this happen so I think they just want you to get the standard professionally installed grid-tied solar system that requires a permit.

This document does not cover using a DIY grid-tie inverters but I just wanted to mention them so you know about them.

How many watts of solar can a 40 amp 12 volt mppt charge controller handle?

AI Overview

A 40 amp, 12 volt MPPT charge controller can handle approximately 480 watts of solar panels, but many modern controllers are designed to "overpanel" and can handle more, up to 600-750 watts, to improve efficiency. The maximum wattage depends on the controller's design, the solar array's open-circuit voltage, and whether it's a standard or overpaneling-friendly model. 

Calculating the power 

• Basic calculation: A simple way to estimate is to multiply the controller's amperage by the system's voltage: \(40\text{\ amps}\times 12\text{\ volts}=480\text{\ watts}\).

• Overpaneling: Many controllers can handle a higher wattage than the basic calculation suggests, often up to 150% of the rated output. This is called overpaneling and increases efficiency, especially in low light conditions.

  • This means a controller rated for 480 watts could potentially handle up to \(480\times 1.5=720\text{\ watts}\).

  • Important: Always check the specific controller's maximum input voltage and power rating, as overpaneling capabilities vary between models. 

Important considerations 

• Check controller specifications: Always consult the manufacturer's specifications for the exact maximum input wattage and voltage for your specific model, as this can vary.

• Overpaneling vs. overcurrent: You can overpanel a controller with higher wattage, but you should not over-fuse the system. The controller's 40 amp limit will prevent it from outputting more than 40 amps to the battery.

• Voltage is key: Ensure the solar array's total open-circuit voltage is within the controller's input voltage limit. 

Wire Size

Summary

For a typical solar system you will need at least the the following three cables:

1. Charge controller to solar panel cable: 10 AWG is typical.

2. Charge controller to battery cable: 10 AWG is typical.

3. Inverter to battery cable: 1/0 AWG is typical. Often the inverter will come with this cable but sometimes the cable that comes with the inverter is not good enough to handle full power.

American wire gauge (AWG) amps

• 4/0 AWG 195 amps.

• 1/0 AWG 125 amps.

• 4 AWG 70 amps.

• 8 AWG 40 amps.

• 10 AWG 30 amps.

• 12 AWG 20 amps.

• 14 AWG 15 amps.

American wire gauge 

Residential 120 volt outlet wire size

• 120V 15 amp outlet uses 14 AWG. 

• 120V 20 amp outlet uses 12 AWG. 

Charge controller to solar panel wire size

Example connecting one solar panel 

To determine what wire gauge to use to connect your solar panels to the charge controller you have to calculate how many amps your solar panels will use. To calculate how many amps your solar panels will use find the watts and Open-circuit Voltage (VOC) of the solar panel you are using.

Assume you are using the solar panel below.

• N-Type 16BB Bifacial Solar Panel 200 Watt,12V 200W Monocrystalline Solar Panel, 25% High Efficiency PV Module, for Van RV Trailer Marine Rooftop Home Power Station Off Grid (1 Pack Silver) ($151.99 7/25/2025)

This solar panel is 200 watts and has an open-circuit Voltage (VOC) of 27.31V volts.

To find the amps you would calculate 200 watts / 27.31V = 7.33 amps so for this solar panel 14 AWG wire is enough.

Even thought 14 AWG is enough for this solar panel many prefer using 10 AWG in case you wanted to expand your solar system in the future.

Example connecting two solar panels in series

If you wire two of these solar panels in series it would double the watts and VOC so the amps stays the same. 

You would calculate 400 watts / 54.62V = 7.33 amps so 14 AWG wire is enough. 

Keep in mind you must keep the voltage below the maximum solar panel (PV) input voltage of your charge controller.

Example connecting two solar panels in parallel

As long as you add your solar panels in series the amps stays the same but sometimes you have to connect solar panels in parallel which increases the amps but keeps the volts the same.

If you wire these two solar panels in parallel it would double the amps and keep the voltage the same.

You would calculate 400 watts / 27.31V = 14.65 amps so 14 AWG wire is enough.

Over paneling is when you supply more amps than the charge controllers amp rating. Most charge controllers can take more amps than their amp rating and will just not use the extra power; however, there is generally a limit of how much you can safely go over in amps.

Can a household extension cord be used to connect the solar panel to charge controller instead of solar wires?

Solar wires can handle higher voltages than typical household extension cords so even if the wire gauge is the same it is not recommended to use household extension cords instead of solar wires.

What battery wire size should you use based on the charge controller watts?

AI Overview

Wire Size Guide for Solar PV Systems (How To Calculate)

The correct wire size for a charge controller to battery connection depends on the charge controller's amperage rating and the wire run's length, but a good rule is to use at least 6 AWG for 60-amp controllers or smaller, and up to 4 AWG for 80-amp or larger controllers. Shorter runs can use a smaller gauge (like 8 AWG for 40 amps), while longer runs will require a larger gauge to minimize voltage drop. 

Factors to consider

• Charge controller amperage: A higher amperage rating requires a larger wire gauge. For example, a 100-amp controller needs a larger wire than a 40-amp controller.

• Wire run length: Longer distances between the controller and battery require a larger gauge wire to prevent voltage drop.

• Wire material: Copper wire is the standard and most efficient. If using a different material, you may need a larger gauge.

• Voltage: In a 12V system, larger gauge wire is always recommended to compensate for the lower voltage. 

Wire size recommendations by amperage

• 30-amp controller: 12 AWG (for short runs)

• 40-amp controller: 10 AWG (for short runs), 8 AWG (for longer runs)

• 60-amp controller: 8 AWG

• 80 & 100-amp controller: 6 AWG

• For longer runs (over 10 feet): Always choose a larger gauge wire. For example, a 40-amp controller with a long wire run may need 6 AWG, and an 80-amp controller with a long run may need 4 AWG. 

Important tips

• Don't go smaller: It's always safe to use a larger gauge wire than what is recommended, but never use a smaller one.

• Fuse appropriately: Add a fuse between the charge controller and the battery, sized at about 125% of the controller's max amperage. For example, a 40-amp controller needs a 50-amp fuse, and the wire should be rated for at least the fuse rating. 

Inverter to battery wire size

AI Overview     

The correct wire size for an inverter to battery connection depends on the inverter's wattage and the distance between the components, as higher wattage and longer distances require thicker wires to prevent voltage drop and overheating. For example, a 1500-watt inverter might use 4 AWG wire, a 3500-watt inverter needs 1/0 AWG, and a 5000-watt inverter requires 4/0 AWG, but a voltage drop calculator should be used for precise sizing, especially for long runs. 

General guidelines based on inverter wattage 

• Up to 1500W: 4 AWG wire is a common starting point.

• Up to 3500W: 1/0 AWG wire is often required.

• Over 3500W: 4/0 AWG wire is typically needed. 

Factors to consider for the correct size 

• Inverter Wattage: Higher wattage inverters draw more current, requiring thicker cables.

• Battery Voltage: A higher voltage battery system (e.g., 24V or 48V) will draw less current than a 12V system for the same wattage, allowing for thinner wires.

• Cable Length: The longer the distance between the inverter and the battery, the thicker the wire needs to be to minimize voltage drop. Use a wire size calculator for accurate sizing, especially for runs over a few feet.

• Current Draw: Calculate the maximum current by dividing the inverter's wattage by the battery voltage (e.g., \(5000\text{W}\div 24\text{V}\approx 208\text{A}\)) and consult a wire size chart or calculator to find the appropriate gauge.

• Safety: Thinner wires can overheat, causing a fire hazard. It is always safer to use a wire that is one size thicker than the minimum requirement. 

Operation

Powerup Sequence

Charge Controller

1. Connect the battery to the charge controller.

2. Connect the solar panels to charge controller.

Inverter

1. Connect the battery to the inverter. Warning, there might be a spark which is normal.

2. If the inverter has a power switch turn the inverter on.

Powerdown Sequence

Charge Controller

1. Disconnect the solar panels from the charge controller.

2. Disconnect the battery from charge controller.

Inverter

1. If the inverter has a power switch turn the inverter off.

2. Disconnect the battery from the inverter. 

Solar System Wiring Diagrams

There are four basic ways to wire a solar system and each is show below with wiring diagrams.

For simplicity I only show a maximum of two solar panels for series and parallel in the wiring diagrams but you can add more than two if you want. However, keep in mind that you must keep the voltage and current within the range limits of the solar charge controller you are using and use the proper wire size.

• Figure 1: One Solar Panel Wiring Diagram

• Figure 2: Two Solar Panels In Parallel Wiring Diagram

• Figure 3: Two Solar Panels In Series Wiring Diagram

• Figure 4: Four Solar Panels In Series Parallel Wiring Diagram

Design Tips

1. 300 watts of solar panels is typically enough to charge a 100 Ah battery in a day; however, it depends on how much sun you get. If you don't get enough sun it will take longer or you will need more solar panels.

2. Recommended battery voltage is; 12V battery for up to 2000W; 24V battery for up to 4000W; 48V battery for up to 8000W. 

3. Decide on your battery voltage first, such as 12V, 24V, 36V, or 48V and buy the inverter and charge controller that can handle that voltage. 

4. You can get a charge controller that can handle more than one voltage which could be useful if you ever want to increase your systems battery voltage. Some can handle 12, 24, 36, or 48 volts.

5. In the future, if you want to change the battery voltage from 12V to 24V or 48V you will need to buy a new inverter because they do not make inverters that can operate at multiple voltages.

6. Remember there is always some loss in the inverter because they are not 100% efficient. Typically an inverter can use 25 watts when idle. To save battery power you can turn off your inverter when you are not using it. Some inverters can automatically turn themselves off when not in use and turn back on where there is a load.

7. In most cases you should get a Pure Sine Wave inverter because it is capable of running any appliance. To save money get a Modified Sine Wave inverter only if you know it will run every load that you want to run.

8. Batteries have a maximum discharge rate. For example, a 12.8V 100 Ah LifePo4 battery generally has a maximum discharge rate of 100 amps or 1280 watts so this means that the most you can get out of it is 1280 watts so if you use a 2000 watt inverter with that battery you can't ever get 2000 watts output. However, if you connect two 12.8V 100 Ah in parallel then you could get more power.

9. 48V is the highest common voltage used in solar systems for residential homes. Also 48V is the nominal voltage so for batteries actual voltage could be 51.2. It is possible that in the future higher voltages might become more common but right now I wouldn't try to go higher than a 48V solar system. If trying to create a solar system with voltages higher than 48V there are many design issues thta need to be taken into account. In addition, higher voltages are more dangerous.

Design Questions

How do you design a complete solar system?

When designing a solar system you need to pick components such as the solar panel, charge controller, battery, inverter, and wires that all properly work together. You can't just mix and max any components and expect things to work correctly. 

For example, if you select a 200W solar panel, PWM charge controller, and 12V lifepo4 battery, there is a good chance the solar panels voltage will exceed the PWM charge controllers Max PV input voltage and could damage the charge controller. In this case if you change the 12V lifepo4 battery to a 24V lifepo4 battery everything might work fine. As an alternative you could keep the 12V lifepo4 battery and replace the PWM charge controller with a MPPT charge controller instead.

I like to start by selecting my solar panels and next what size battery and select the charge controller last.

How many solar panels and batteries are needed to run a standard refrigerator?

My estimates will only account for exactly what you need assuming no days without sun. If you want to add a safety factor for days without sun you can add that later. For example, if you want a safety factor of 3 days without sun then take the results and multiple times 3.

Estimate of Wh used 

In this estimate I am using the Mora 18 cu. ft. Top Freezer Refrigerator from Costco that cost $579.99. I am using the Energy Guide estimate of 403 KWh per year. 

403 KWh per year / 12 = 33.58 KWh per month / 30 = 1.12 KWh per day = 1120 Wh per day.

The inverter will use power just while running idle and not running a load. A 1000 watt inverter can use about 20 watts of power continually. This is 20 watts x 24 hours = 480 Wh per day.

• Refrigerator: 1120 Wh per day

• Inverter: 480 Wh per day

• Total: 1600 Wh per day

Estimate of battery in Ah needed

1600 Wh / 12.8V = 125 Ah 

So you need a 12.8V 125 Ah. I am going to round this down to 12.8V 125 Ah because that is a common size battery. If it turns out not to be enough you would need to add another 12.8V 100 Ah battery.

Estimate of solar panels needed in watts 

You can use the website below to find the lowest average sun hours per day for the year in your area or you can just guess. A good guess would be 4.

https://pvwatts.nrel.gov/

I am going to use 4 hours of sun per day on average.

• 1600 Wh / 4 hours of sun = 400 watts

So you need 400 watts of solar panels.

Conclusion

To run a standard refrigerator you will need:

• Solar panels: 400 watts 

• Battery: 12V 100 Ah LifePo4 

Note these values don't include a safety factor for days without sun.

How many solar panels and batteries are needed to run a mini refrigerator?

My estimates will only account for exactly what you need assuming no days without sun. If you want to add a safety factor for days without sun you can add that later. For example, if you want a safety factor of 3 days without sun then take the results and multiple times 3.

Estimate of Wh used 

In this estimate I am using the Danby 3.3 cu ft Compact All Refrigerator from Costco that cost $229.99. I am using the Energy Guide estimate of 36 KWh per year. I am using my cost of electricity in California of about 0.50 per KWh.

The inverter will use power just while running idle and not running a load. A 1000 watt inverter can use about 20 watts of power continually. This is 20 watts x 24 hours = 480 Wh per day.

36  KWh per year / 12 = 3 KWh per month / 30 = 0.1 KWh per day = 100 Wh per day.

• Refrigerator: 100 Wh per day

• Inverter: 480 Wh per day

• Total: 580 Wh per day

Estimate of Ah battery needed 

• 580 Wh / 12.8V = 45.3 Ah 

So you need a 45.3 Ah battery. I am going to round this up to 100 Ah which is more than you need but I think the 100 Ah battery size is more popular. If you want to save money you can go with a 50 Ah battery.

Estimate of solar panels needed in watts 

You can use the website below to find the lowest average sun hours per day for the year in your area or you can just guess. A good guess would be 4.

https://pvwatts.nrel.gov/

I am going to use 4 hours of sun per day on average.

• 580 Wh / 4 hours of sun = 145 watts

So you need 145 watts of solar panels. I am going to round this up to 200 watts.

Conclusion

To run a mini refrigerator you will need:

• Solar panels: 200 watts 

• Battery: 12V 100 Ah LifePo4 

Note these values don't include a safety factor for days without sun.

Solar System Tips

• LifePO4 batteries should not be charged when temperatures are at or below freezing to prevent battery damage. Some batteries come with a low temperature shut-off to protect the battery from charging during freezing temperatures. Some batteries even have built in heaters to keep the battery warm in freezing temperatures.

• The wires running from the battery to the inverter should be as short as you can make them so that power loss from the wires is kept to a minimum. This is even more important with lower voltage batteries (i.e. 12V) because they run at higher amps.

Solar System Future Trends

• Using EV car batteries for home battery storage is a trend likely to become common in the future. EV cars have powerful batteries capable of running an entire house and if you are going to have an EV car anyway it is like getting a free battery for your home solar system.

• In 2024, Solid State batteries are starting to become available for DIY off-grid solar systems; however, it is not yet clear if they will be better and cheaper than LifePO4 batteries.

• In 2024, Sodium Ion batteries are starting to become available for DIY off-grid solar systems; however, it is not yet clear if they will be better and cheaper than LifePO4 batteries. Some say the discharge curve is not anywhere near as good as LifePo4 batteries so keep that in mind when evaluating Sodium Ion batteries.

Solar System Abbreviations

• A: Amps

• AC: Alternating current 

• Ah: Amp Hour

• BMS: Battery Management System

• DC: Direct current

• I: Current

• kWh: Kilowatt-hour

• LiFePO4: Lithium Iron phosphate

• MPPT: Maximum Power Point Tracking

• PWM: Pulse Width Modulation

• V: Volts

• W: Watts

• Wh: Watt hours

Solar System Formulas

• Convert amp hours to watt hours: Wh = Ah × V. (Example: 1280 Wh = 100 Ah * 12.8V)

• Convert watt hours to amp hours: Ah = Wh ÷ V. (Example: 100 Ah = 1280 Wh ÷ 12.8V)

• Convert Watts Into Kilowatt-Hours: kWh = Wh ÷ 1,000. (Example: 1.28 kWh = 1280 Wh÷ 1,000)

• Calculate Wh: Watts of device × Total hours used (Example: 7500 Wh = 750 W× 10 hours)

• Convert Wh to kWh: kWh = Wh ÷ 1,000

• Grid Electricity Total Cost: kWh used × your kWh rate.

• W (Watts) =V (Voltage) × I (Current)

• I (Current) = W (Watts) ÷ V (Voltage)