What size LED grow light do you need for growing cannabis indoors? Unlike the world of Metal Halide (MH) and High-Pressure Sodium (HPS) lighting, LED lighting poses a bit of a mathematical challenge for rookie growers setting up their indoor grow space.
Wattage was the conventional approach to calculating the number of grow lights in a grow tent, but it doesn't easily convert into LEDs' language. When it comes to LED Indoor Grow Lights, wattage is not always a great conversion.
Instead of strictly relying on wattage, you'll need to look at a few factors to make an accurate estimate for the number of LED grow lights you'll need and the appropriate size.
What is a HPS Light?
Invented in 1964, HPS stands for High Pressure Sodium. It’s a light technology that has shaped how we cultivate indoor plants in a huge way. HPS bulbs consist of a narrow arc tube supported by a frame in a bulb. To improve efficiency, the arc tube is kept at an extremely high pressure. Sodium, mercury and xenon are usually used inside the arc tube in order to improve the light spectrum.
The most common way to start the bulbs is with a pulse start. This pulse starts an arc through the xenon gas and the lamp turns sky blue. The arc then heats the mercury which then lights up, giving the bulb a bluish color.
As the bulb heats and the sodium starts to vaporize it strikes an arc over 450° F (240° C). The sodium is then mixed with other impurities to create a more “white” light. Mercury helps add a blue spectrum light to the pure yellow of the sodium.
Why Are HPS Lights So Popular?
Usually HPS technology is chosen because it is a powerful light that provides a spectrum that plants can use well, specially for flowering, blooming, and fruiting crops. HPS bulbs also come in many different sizes, making them a flexible pick for your indoor garden.
Although there are 150 watt bulbs and 250 watt bulbs, those usually come with their own ballasts and reflectors in an all-in-one kit. When it comes to bulbs, the most commonly used sizes are 400W, 600W and 1000W.
As a general rule, 400w HPS bulbs are the most common, 600w HPS bulbs are the most efficient, and 1000 watte HPS grow lights are the most powerful (but also the hottest).
Before we continue, here’s a rundown of the good and bad points of running an HPS grow light:
The Good
Good spectrum for flowering and fruiting
Affordable and reliable
High lumen output at 80-160 lumens per watt
High PAR efficiency when compared to fluorescent lights
Different sizes for different spaces
The Bad
They produce a LOT of heat and need cooling
Suffer significant degradation over time
Require a ballast and reflector
HPS Spectrum Output
Different grow lights produce different spectrums of light. Plants grow responding to the color spectrum of the light they’re receiving, this process is scientifically known as photomorphogenesis.
HPS bulbs emit a golden-yellow light that is specially good to maximize budding and flower production. Their spectrum what makes HPS bulbs emit their respective golden colored light.
Here’s an example of what the usual HPS PAR spectrum looks like:
HPS Spectrum
Spectrum of the average HPS bulb.
Note that we said “PAR spectrum.” PAR stands for Photosynthetically Active Radiation and is light between 400 and 700 nanometers which is specifically used by plants at certain ratios for photosynthesis. HPS bulbs also produce a little light outside of the PAR spectrum, like infrared but this light is generally not useful for our plants and greatly contributes to heat production.
HPS bulbs tend to produce most of its usable light as yellow and red light. This is why some growers choose to supplement their HPS grow lights with Metal Halide (MH) lights which have a green-blue dominant spectrum for more balanced light spectrum.
Choose Your Bulbs
MH and HPS bulbs will need to match your ballast. If your ballast supports 600W Metal Halide, you need to get 600W Metal Halide bulbs. If your ballast supports 1000W High Pressure Sodium, you need to get 1000W High Pressure Sodium bulbs.
Some ballasts support both Metal Halide and High Pressure Sodium, and will clearly state that on the box.
Some ballasts are digital and dimmable, which means you need to get bulbs that support digital/dimmable features.
Some bulbs claim to be “enhanced” or be ultra-efficient. While there is not a whole lot of difference between the different bulbs, you will tend to get slightly better results from the higher quality bulbs, and they tend to be more durable/long-lasting.
Therefore, like with the ballast, I recommend getting the best bulbs you can afford. These bulbs generally last 1-2 years of normal use, so you will not have to replace them for quite a while. They do get dimmer over time, so it’s a good idea to always replace all your bulbs every 3-4 grows, even if they haven’t burnt out yet.
Never buy used bulbs!
Some sellers will actually replace burnt out bulbs within a certain time period, so definitely take advantage of new bulbs with guarantees if you can!
For longer lasting bulbs….
The biggest wear to HID light bulbs come from turning the lights on and off, as opposed to how long they run. The worst thing you can do as far as wear and tear is turn off an HID light and then turn it on again before it’s had a chance to fully cool off.
Sometimes bulbs blow right away, and the most common reason this happens is because the type of bulb doesn’t match the type of ballast you have. So make sure your ballast matches your bulb in every way that’s listed on the packages. If your ballast says “magnetic” your bulb should say “magnetic.” Getting a digital ballast with your grow light system will lower the chances you run into problems with incompatible bulbs, since they’re compatible with the most kinds of bulbs.
If you’re using a non-standard ballast (for example a 150W or 250W HID grow light with the ballast and hood built in instead of separate), it’s a good idea to double-check the specifications of your ballast just to double-check it doesn’t need some special kind of bulb. When you buy your bulb and ballast together as part of a set, you generally don’t need to worry about incompatibility, and you can make sure to always get the same type of bulbs if you ever need to replace it.
Always double-check to make sure you have a way to return or exchange a bad or malfunctioning bulb, especially when purchasing bulbs from a small local shop or unfamiliar online seller
Choose Your Hood
There are many different kinds of hoods for HID lights, from wings to cooltubes to air-cooled hoods. The hood will reflect the light from the bulbs down at your plants, and may come with special cooling features, too.
In my experience and from talking to all the growers I know who’ve experimented with different hoods, many growers seem to agree that air-cooled hoods, especially extra wide air-cooled hoods, get the best yields and results when growing marijuana.
There are a lot of factors that can influence your decision. You choose the best HPS grow light according to the size of your garden and your own needs.
Like l mentioned, I advise you to start with HID grow light kits. I hope that these reviews will help you to make the best conditions for your plants.