Choosing the right LED grow light can be confusing without some background. Here we’ll go over what to consider before buying your light so you know what questions to ask in order to get the perfect light for your garden.
LED naming conventions can be misleading. Something called an N400, for example, has nothing to do with wattage so you’ll need to do some digging to find the running wattage, which is how much the light pulls from the wall socket.
Keep in mind that there are a lot of 5 watt LEDs on the market that are really only pulling two watts. They look to be 3 watt LEDs just being overdriven.
The other thing about LED wattage is you need to watch out for LEDs labeled as having high-powered chips but are really being driven at low wattage.
Determine Your Watts Per Foot
One of the first things to check is watts per foot. In any grow room, you’re going to have a target watt per foot ratio. With high intensity discharge (HID) lighting, like high pressure sodium (HPS) lights, it’s always been 50 watts per foot. That’s industry-standard. With LEDs, you’re looking usually at about half of that.
What’s recommended is 30 watts to 35 watts per foot for LEDs. So that’s a key indicator – watts per foot. With HID you’re looking at 50, with LEDs you’re looking at around 30 to 35.
Start first with watts per foot to get the total wattage that you need for your space. Here’s an example.
You have a 3 foot by 3-foot space and you need 30 plus watts per foot. So it’s 270 watts or roughly 300 watts. So first you’re going to look for a 300 plus watt LED to cover this space.
Get LEDs with Secondary Optics
Now here’s where you need to consider secondary optics, which some LEDs have and some don’t. There’s a myth that secondary optics reduce the light output and the troubling thing is it’s both true and false.
It’s true because any kind of material put over an LED reduces the total light output. That’s true. But the difference is if you take a secondary optic lens like total internal reflection (TIR) built for an LED, you actually get more light energy to your plants.
If you’re covering a three by three foot space and you have no secondary optics – it’s just the bare LED pointing at the space, the light is going to spread out and you’re going to lose a lot of energy.
Light reflected from the walls doesn’t help your plants. The energy is depleted from scatter, absorption and from it just traveling back and forth.
You need to get the light from the LED straight to the plant canopy in the shortest distance possible.
It’s just like a fire hose or a garden nozzle. If you add a secondary optic, it’s like turning that water nozzle; you’re focusing that energy through that LED right to the canopy space that you want in the shortest distance to your plants.
So you want to maximize your light energy or more specifically your photosynthetically active radiation (PAR) which is light in the wavelengths from 400 to 700 nm. It’s the part of the light spectrum absorbed by plants for photosynthesis. You measure PAR with a spectrometer.
So with a secondary optic you may lose 10% of your light energy, but you’ll increase your PAR by 50% or 100% inside of the canopy space.
Choose Secondary Optics with the Correct Angle
It’s best to use secondary optics of 90 degrees, avoiding 60 degrees or less. Some LED lights have a 45-degree optic on there to really focus the light, but the problem is you end up with core coverage so tight and small you are not efficiently covering the entire plant canopy.
90 degrees is perfect because if you’re a foot and a half away, you’re getting a three-foot coverage. If you’re two feet away from the plant then that means you’re getting four-foot coverage. You calculate the coverage by multiplying the distance by two with a 90-degree optic.
Not all secondary optics are the same. The best ones give you a good even coverage across a 90-degree spread. And then you can get fixtures with a reflector to focus wasted energy.
Another big benefit of having a secondary optic is penetration of your plant canopy. If your plants are 3 feet tall and you’re not sending light straight down, you’re losing penetration and limiting the growth of your plants.
Figure PAR Per Watt for Efficiency
PAR per watt is a key indicator of the efficiency of your light. Getting at this number is tricky because different manufacturers measure PAR in different ways. One way is measuring only center PAR. That’s when you take your light source and you measure directly underneath it.
The other way, which provides a better picture of things, is taking the total PAR over a grid, say a four by four foot grid. Then you can divide that by the running watts from the wall to get total PAR per watt.
For example, a 1000 watt HPS grow light has a very low PAR per watt of perhaps 6 or 7. That’s because it’s very inefficient and produces a lot of infrared.
Select the Best Spectrum
Spectrum is the final consideration.. To choose the right spectrum you first have to determine what type of plant you’re growing. If you have a vegetative cycle or a vegetative plant you want mostly blue spectrum. You still want it to be somewhat full but blue is the most important.
If you’re using a heavy flowering or fruiting plant, you want a lot of red and you need a lot of PAR. Keep in mind that in the green range, plants aren’t going to absorb as efficiently. So you want to look for the blue and the red and you want to look for the maximum amount of PAR per watt.
Choosing the right LED grow light requires digging up some data and running a few calculations. You’ll want to figure out how many watts per foot you’ll need, pick LEDs with secondary optics featuring a 90 degree angle, determine efficiency with PAR per watt and then choose bulbs with the right spectrum.
After you’ve done your homework, you’ll have the information you need for choosing the best lights for your grow room and maximizing the productivity of your crop.