Wondering what size led grow light do you need? Here How to Calculate how many watts per square foot for led grow lights your plants actually require to provide excellent growth.
Setting up your own grow space? Perfect!
A great growing system is composed of the appropriate room size, soil, plant density, relative humidity, the right temperature, proper air circulation, and the perfect LED light.
But this where things can get tricky. You might have heard that wattage will dictate the light’s power.
While this is partly true, wattages do not exactly equate to power output. Although, wattage is one way to establish the light output needed for your plants.
What matters is the amount of usable light reaching per square foot of your growing area.
Generally, your goal with LED panels should be 32 watts per square foot of a grow space to get the best results. However, this would still depend on your growing conditions.
You have to determine the size of your grow room, consider PAR and PPFD levels, and identify plant density to secure the Correct LED Wattage for Your Grow Space.
Sounds like full of numbers and acronyms? Don’t fret, we have simplified calculations in this post so you can easily convert the light intensity you need in watts per plants.
We’ve also included tables so you can see how many watts per square foot for led grow lights you actually need, actual grow space size, and different growth stages.
By the end of this article, you will gain knowledge on how to select the right watts of LED grow lights per square foot of your grow space.
How to Calculate the Correct LED Wattage for Your Grow Room
To establish the correct amount of wattage, you must first identify the size of your grow room and multiply it to your desired PPFD level. The output should then be divided by the PPFD per watt of your LED grow light.
Note that wattage shouldn’t be the sole determiner of your coverage area nor should it dictate the effectiveness of a LED grow light.
Basically, wattage is used to identify the approximate correct fixture size for your grow space. It should only guide and determine how much power your LED grow light can give per square foot of your grow room.
In fact, Photosynthetic Photon Flux Density or PPFD (μmol/j/m2) and DLI (Daily Light Integral) are the appropriate metrics for measuring light intensity over a specific grow area.
Aside from the correct light spectrum for your marijuana type and growth stage, PPFD is another essential factor for growth.
Hence, product specifications showing LED wattage pertain to the maximum LED wattage and the number of LEDs.
- 300 x 3W diodes = 900W LED grow light
Yet, that product might only draw 500W at the wall.
Meanwhile, there are cases wherein your LED grow lights could have a high wattage, but a minimum light output.
This is where PPF and DLI come in. Photosynthetic Photon Flux or PPF measures the total amount of light emitted by a light source per second. It also tells how much PAR is created.
On the other hand, DLI (mol/m2/d) measures the total amount of light a plant receives on a daily basis. It measures the number of moles of photons per square meter a day.
- 2.5 μmol/j x light output of 500W = PPF of 1,250 μmol/s
- 2.0 μmol/j x light output of 600W = PPF of 1,140 μmol/s
As you can see, the second model has a bigger wattage, but is 100 μmol/s shorter.
So when you see a LED lighting fixture that states 600W or 1000W, this typically refers to the diode wattage, but not the definite power draw.
This is why it’s vital for you to only consider the stated wattage as a deciding factor in the light output needed per square foot of a marijuana grow space.
1. Determine the Size of Your Grow Space in Square Meters
Your actual grow room space is a critical aspect in determining the wattage needed per square foot.
First, you have to take the width of the area in meters and multiply it by the depth in meters.
Then, simply follow this formula: multiply the width by the depth in meters.
- 4 foot x 4 foot x 1.2m x 1.2m = 1.44 square meters
Normally, 1 square foot of a marijuana grow room needs about 32 watts of actual wattage.
Here’s a table showing the usual wattage for different sizes of a grow space.
6x7 (40 sq ft)
2. Decide on Your Light Level in PAR Intensity
Urban gardening has been made more effective by the most accurate measurement for plants, which is the Photosynthetically Active Radiation (PAR).
PAR is the light spectrum that marijuana plants need for photosynthesis. It is a measure of the energy discharged from a light source that is used by plants for photosynthesis and growth.
Therefore, the higher the PAR output of your LED grow light, the more edible light your plants get.
Usually, the PPF is above 1100 µmol/m2/sec or so, which can cover a 4x4 grow area. Nonetheless, the optimal PAR output should still depend on the growth stage of your plants.
There are various rates of growth connected with different levels of power.
Moreover, PAR is the range of light, including red, blue, and green wavelengths. These wavelengths are typically in the ranges of 400nm to 700 nm.
Plant growth rate surges up to PAR levels of 1,500 µmol/m2/sec at atmospheric levels of carbon dioxide and optimum temperature.
However, the high level of power for good growth rate and a bountiful yield is 750 micromoles per meter square per second. Increasing the PAR output beyond this level won’t equate to a boost in growth rate.
When measuring in PAR, all light wavelengths or colors are considered to photosynthesize at the same rate.
3. Select the PPFD Per Watt of Your LED Grow Light System
As I’ve explained earlier, PPFD measures the light that reaches a plant each second. It is measured in micromoles per square meter per second or μmol/m2/s.
Similarly, PPFD is one of the most crucial numbers for a LED grow light system.
Here’s a table showing some of the major marijuana light systems and their associated efficiencies or PPFD per watt.
- 0.9 – 1.2 PPFD per watt : Budget Red/Blue
- 1.3 – 1.5 PPFD per watt : High Spec Red/Blue
- 1.2 – 1.5 PPFD per watt : Full Spectrum LED
So, going back to step 1. We found that if your grow room in square meters is 1.44, we must multiply it to the desired PPFD level of 750 μmol/s.
- 1.44 x 750 = 1,080 PPFD
The PPFD output should then be divided by the PPFD per watt of your LED grow light. For this scenario, we’ll put it at 1.4 PPFD per watt.
- 1,080 / 1.4 = 770 watts
However, note there are still other factors to consider such as your plant density and lighting accessories.
Lindsey Hyland grew up in Arizona where she attended University of Arizona’s Controlled Environment Agriculture. She has supplemented her formal education by working on various organic farms, including spending a semester abroad in India.
Growing and/or raising just about anything gets her excited. She is especially passionate about environmental justice and low-tech, sustainable ways to better run small-scale farms and homesteads. Lindsey started Urban Organic Yield to discuss gardening tips and tactics.