Determining how many grow lights you need is one of the first steps to setting up your grow room, and its important you get it right the first time so you don't waste time, energy, and money.
A grow light coverage calculator can make a seemingly complex topic simple.
Once you know how much light your plants need and what type of grow lights you are going to use, it's just a matter of choosing a grow light setup that works for you.
Calculate your grow light requirements first
Calculating your grow lighting requirements really is just two steps - determine your plant’s lighting needs (watts/square foot), and your plant canopy dimensions.
A common mistake new growers make is measuring the dimensions of the grow room itself. What you really should be measuring is the footprint of your plant canopy.
For example, if you are growing in a 5’ x 5’ space but your plant canopy only takes up 4’ x 4’, you don’t need a grow light capable of lighting the 5’ x 5’ space - it would be overkill.
How to calculate grow light wattage requirements
Cannabis plants grow best with 20-30 watts of light per square foot. Don’t make the mistake of thinking more is better. Too much light can lead to bleaching and burning your plants.
Let's say you are growing in a 5’ x 5’ grow space. If your plants need 20-30 watts/square foot, and your canopy takes up 4’ x 4’ of that space, you will need 320-480 watts of grow lighting.
How to calculate LED wattage requirements
If you are planning on using LED grow lights, there are a few things you need to know about calculating your grow light coverage.
Some manufacturers have two different wattage specifications - displayed wattage and actual wattage (power draw).
Because LED grow lights operate so efficiently, manufacturers will display a wattage equivalent to HPS. For example, the Gavita LED 1700e is 646 actual watts, but is equivalent to a 1,000 watt HPS.
So, if your 5’ x 5’ growing space requires 320-480 watts of grow lighting, and you choose the LED route, make sure your LED light features an actual wattage of between 320-480.
So how many lights do I need?
Now that you know how many total watts of grow lighting you need, you can decide how many lights you will use. Our grow light setup post will help you with this step, as there are plenty of possibilities.
With this 5’ x 5’ grow example, the space is big enough where two grow lights could be an option. However, it would make more sense to use one large grow light.
The number of grow lights your room will need really depends on the size of it. Grow lights have gotten more and more powerful as technology continues to advance, but at a certain point, 2 lights can cover an area more effectively than 1.
In a smaller grow, say 2’ x 2’, it really only makes sense to use one grow light.
If you have a perpetual grow, matters can get further complicated. You will need lights for cloning, veg, and flower chambers.
As long as you use our grow light coverage calculator and give your plants the wattage they need, the number of lights is up to you.
As we explained, there are certain grows that by design will use more or less lights, so it is up to you to use your best judgement.