In the red box are the amp loads for each of our 12v loads when turned on. We measured them with our simple battery voltage/ammeter that we use to monitor the battery. It has a shunt so it gives us amps (but does not really integrate amps going in and amps going out to give us a good "% full" indication).

Yellow shaded areas are for inputs. Green shading indicates calculated values so no entries can be made there. For the AGM battery that came with out camper in 2019, we enter 1 for AGM battery type and 79 amp-hours for its rated capacity.

Enter "2" here for lithium batteries. LiFePO4 batteries are most common for RVs. These are Lithium-Iron-Phosphate batteries.

The actual capacity of a battery in amp-hours is not necessarily the same as it's rating (see "Battery 101". For AGM batteries it is about 50% of the battery rating (i.e. 39.5AH for a 79AH rated AGM battery). For lithium batteries the useable capacity is 80 to 90% of the rating so you get much more useable power from a lithium battery as you do with an AGM with the same rating. I'm conservatively using 80% for Lithium batteries here.

Some loads cycle on and off with a thermostat control. These include your refrigerator and your furnace. So their measured amps when running must be reduced to reflect that they are not on all the time. Under typical temperature conditions, I have found that our refrigerator cycles "on" about 20 to 25% of the time. So 25%, its average amps are only (3.0 x .25) or 0.75 amps. The cycle time for a furnace varies depending on how cold it is, how much ventilation, etc., and I am not sure, but I'll estimate conservatively, that it is on 40% of the time.

This column is where I enter how much we use each device in a typical day. Some devices, like the refrigerator and CO/LP dectector run 24 hours a day. However on most days from April through October we typically only turn on the furnace for about 15 minutes (0.25 hr) in the morning to take the chill off when we crawl out of bed. We might use the fan on Setting 1 for cooling one hour and use it on Setting 3 to exhaust steam for 6 minutes (0.1hr) when we make coffee. And we use some lights in the evening. But we are outside most of the time and don't really use much power for other purposes. We don't have a a 12v water pump and I don't know how much it draws and for how long so that is not in the table.

Our daily battery charging cycle begins at sunrise when the battery voltage is lowest, then it peaks when the sun is going down and hits another minimum battery charge after running all night and just before sunrise. We may run some things like the refrigerator all day long, be we know our battery is at its maximum charge at the end of the solar day and really only needs to support 12v power needs from that point until the sun starts powering the panel the next morning, usually about 12 hours. This might be only 50% (0.5) of the refrigerator and LP/CO detector use, but since we only use the lights at night, 100% of their use must be supported by the battery alone.

However, if we camp in a dense forest or it is overcast all day and we are not driving to charge the battery with the alternator, the battery must carry all the loads for 24 hours. In that case we change all these factors to "1".

After all the yellow-shaded entries are entered, they are all multiplied together to calculate the total amp-hours used for each device in one day. These values are shaded green. You can see our refrigerator is typically our biggest load.

The "AH Used" are summed to determine the total amp-hours used for the day for which the battery must supply power. If the "Fraction of Use after Sunset" has any values less than "1", there are additional amp-hours being used (during daytime), but they really don't add to the total usage that the battery must support when the solar charging ends with sunset. By doing this, we get to the answers we are looking for: "does my battery have the capacity to get me through one full day". One takeaway here is to use discretionary loads early in the day to allow the battery to fully charge as the sun goes down.

After we get the total amp-hours that the battery must support overnight, we divide it by the amp-hour capacity, in this case 39.5 amp-hours, indicated at the top of the green column, second row. In this typical case we are projected to be at 36% of capacity so there is plenty of juice left.

In order to compare a 36% state of charge to the resting voltage that we should see early the next morning (before the sun hits the panel) we need to go to the chart we described in "Battery 101" showing "Resting Voltage vs % of Battery Used Up". It is shown again below.

To use this chart, you can find a point on the line for 36% of "Battery Capacity Used Up", then go up to the line to find a resting voltage around 12.57v. In the amp-hour budget, we use the equation shown (Y=-0.5x + 12.75), where "Y" is the battery resting voltage and "X" is the % of Battery Capacity Used Up".

In other words "% of Battery Capacity Used Up" times 0.5 volts tells us how big the drop in resting voltage will be. So subtract that from the resting voltage at full charge (12.75v) and we get the expected resting voltage.

If you are camped in a dense forest or on an overcast day, you will get little or no charge to your battery. If you are travelling, the alternator charging will help, depending on what gauge the wire to your camper is. Assuming you are not driving, you will get no charge and your battery must carry the amp load for 100% of the day, so set the "Fraction of Use After Sunset" to "1" for all loads.

Doing this gives you the actual total amp-hour load for your camper in 24 hours.

If you want to look at 2 days without sun, you have to multiply the total amp-hour usage from the 24 hour case above (25.6) by "2" to get 51.3 amp-hours. In version 2 downloaded below, there is a place on this line to enter number of days without sunshine that you want to simulate then the multiplication is done automatically.

Now we can see that after 2 days with no sun we will be at 130% of battery capacity and the resting voltage will be dangerously low, reducing the life of your battery. I would be shutting things down before getting that low.

We recently upgraded to a 105 AH AGM battery. So if I change the rated capacity from 79AH to 105 AH, we get 52.5 AH capacity and I can see how the new battery will handle the 2 days without sunshine. Now we find that 98% of our capacity is used up, so on the rare occasion when this occurs, we will still be ok with the new battery. And that is good enough for me.

Now, if we had upgraded to a 100AH Lithium battery, we would use at least 80% of its capacity or 80 AH, and you can see that we would end up, much more comfortably, at 64% of battery capacity. That would be nice, but not worth the up-front cost for us, since 2 days without sun is such a rare case, and we can easily conserve on such occasions.

Note, I don't have data on lithium voltage vs state of charge so I won't try to figure out the final resting voltage for lithium.

Above, we covered our typical case for most of the year. However, we try to camp at least once in every month and our usage goes way up in the winter. We use more power in the winter because it gets dark much earlier and we spend a lot more time in the camper using the lights and furnace.

In this red box, I've estimated winter time "Hours of Use per Day". As you can see, in winter we used 79% of our battery capacity with the old 79AH battery instead of 36%, or about double.

This scenario looks at the worst case winter day when when overcast or shaded conditions also blot out the sun all day. In this case the 79AH AGM battery is at 103% of capacity and the battery voltage, at 12.24v, is about the lowest you ever want to see. It will go lower, but that will reduce your battery life expectancy.

Also, due to the surface charge it may read much lower under load and that can get scary! Calm down, turn things off and read it again after 20 - 40 minutes.

Now, let's look at the same overcast winter day with my new 105AH AGM battery. The battery ends up at 77% capacity and 12.36v resting voltage. I feel much better and since we rarely camp for more than one day at a time in winter conditions, we feel that the 105AH AGM is just fine for us.

Let's say you are more rugged than us and plan to camp for multiple days in the winter, then end up enjoying 2 days without sun. As you can see to the left, you will just make it with a 100AH lithium battery! But, if you camp longer-term in overcast/shady spots in the winter/early spring you should probably look at something more like 200w solar and 200AH of lithium batteries. Of course if you have more amp loads than we do, you will want to scale up these results accordingly, or construct your own amp-hour budget to include all your appliances and devices.