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.
2x3 (6 sq ft)
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.
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.
How Many LED Grow Light Watts Are Needed Per Square Foot of your Grow Space?
Read on to discover how these may impact the watts needed per square foot of your grow room.
Consider The Number Of Plants You Are Growing
It could be specific numbers like 1, 2, 5, 10, 25, 50. Yet, indoor cannabis growing has styles you can use.
Generally, experienced cannabis growers use this table:
1 plant per square meter
This is done by weaving cannabis shoots through a screen to utilize grow space.
Pruning is a yield-boosting technique by allowing plants to bush out.
Since this is a beginner-friendly technique, a user is allowed to manipulate a plant to grow like a bush.
As you can see, the watts would still depend on how densely plants are placed, as well as how high the LED grow lights are placed at 1 plant per square foot. Usually, a 300W LED grow light should be able to serve 8 plants.
Typically, each weed plant needs at least 1 square foot of space. Most LED grow lights can cover about 1-6 plants.
If you want to have more plants, I suggest placing multiple lights side by side. As a result, you get to cover the canopy, guaranteeing each plant is receiving sufficient light.
How many plants and how big LED’s?
Here’s the catch—LED grow lights have specific wattages. For example, a certain model could have 3W, but this will easily burn out when you run the unit at 3W electric power.
You might be enticed by a certain model that brags about 600W. Yes, that’s impressive.
However, it could probably only draw about 240 to 260 watts of electricity to obtain the same effect as a 600W HPS or HID grow light.
Thus, it is best to operate LED grow lights on 60% power. This is also applicable when you follow the 4 square foot per plant rule.
But looking at this table, you’ll see that the power of a LED lighting system increases depending on the size of your grow room and plant density.
4 sq ft for each plant
Going back to our introduction, we shared that LED panels should be 32 watts per square foot of grow space to achieve the best yields.
For example, you have 5 plants in a 6 square feet space.
5 x 6 = total footage of 30 square feet
Following this rule of thumb:
30 square feet x 32 watts = 960W LED grow lights
Another situation: you have 2 plants in a 4 square feet space
2 x 4 = total footage of 8 square feet
8 square feet x 32 watts = 256W LED grow lights
Here are other samples if you have different plant numbers and area size.
(1) 120-140W unit
(1) 270W-310W unit or (2) 140W units
(1) 750-1000W unit or (4) 180-240W units
The good thing about LEDs is that 95% of the energy goes into the light while only 5% is used on the heat emission.
This means that even if you have a bigger grow space, you ensure the wattage is powerful enough to cover your marijuana canopy.
Would the Height of a LED Grow Light Impact the Watts?
Absolutely. However, be aware that the distance between the lights and the canopy has an impact on light intensity.
Likewise, using powerful LED grow lights and keeping them in the right distance are some of the ways to achieve bigger yields.
Remember not to keep your LED lights too close to your plants to avoid bleaching, nutrient deficiencies, sunburn, and appearance of yellow leaves.
In reality, there’s no standard distance required to place each LED grow light, but there are recommended grow light height from plants on each growth stage.
Sometimes, the distance may also depend on the model and the brand. Here’s a quick reference you could follow:
If you have the extra money to do so, why not? Using light reflectors is actually a cost-saving method because it focuses the light coming from a bulb by preventing light from scattering around.
Yet, not acquiring a good reflector is a waste of money.
There are five sizes of light reflectors: compact, large, rectangular, parabolic, and square.
In addition, there are four types of reflectors that can enhance the wattage output of your LED grow lights:
Hood – A hood reflector delivers the best intensity for marijuana plants by directing the light from a bulb down to the canopy. They are appropriate for urban gardening because it intensifies light, which ensures good canopy penetration.
Wing – This is the most popular type of reflector because they can effectively provide a focused yet wide spread of light over your marijuana plants. A wing reflector also has a decent focus despite being cheap.
Umbrella – This type of reflector is ideal when you are covering a large area, but won’t offer lots of canopy penetration. Due to its circular shape, it may not deliver lots of focused light, but it permits the bulb to shine in an open environment.
Air-Cooled – This reflector is another favorite because of its ability to lower the heat coming from the bulbs. However, this type is not preferable when covering a huge area.
How to Pick the Right Luminaires?
Pairing a light reflector with the right luminaire is another booster to your wattage per square foot, and eventually, marijuana yields.
Owning good luminaires help you acquire more efficiency even from the same old LED grow lights you own.
Typically, there are three sorts of luminaires used:
Tube LED grow light – Produces low intensity, mostly for small grow spaces, vertical farming, and tissue culture. It is often used as a substitute for T8 fluorescent tubes.
Bar LED grow light – Produces varying intensity, but can cover a wide area of cannabis plants. They can be placed high on multi-tier cultivations and medium grow spaces.
Box LED grow light – Produces ultra-high intensity lighting. They are normally used to cover large numbers of plants in greenhouses.
You can choose among the three once you’ve determined the precise amount of light your marijuana plants need.
The application of these luminaires ranges from 10-100 µmol/m2/s to 2000 µmol/m2/s.
Should I Consider the Chip Size?
Yes, the chip size is also a contributing aspect to the wattage per square foot. LED grow lights come in various chip sizes, with the usual ones ranging from 0.25 watts to 10 watts.
Larger chip sizes are now available for a LED lighting system. Yet, a 3-watt chip remains the favored size by urban marijuana growers.
A 3-watt chip balances heat dissipation and light penetration into the cannabis canopy. It is large enough to penetrate 5 to 6 feet into the canopy.
For marijuana growth purposes, I highly recommend using a 3-watt chip to provide good penetration at an affordable price.
The downside is that the bigger the chip size, the larger the amount of heat becomes present.
Growing cannabis indoors may need a lot of energy and power consumption. Plants need a light and energy source that can emit the rays similar to the sun.
Planning a good LED lighting system is essential for a healthy marijuana development.
Remember that wattage alone won’t ensure the light output of your LED grow lights. Wattage should give you an idea about how powerful your LED lighting system could be.
It is important you are able to determine the size of your grow place, as well as how many plants you intend to grow per square foot.
The wattage should then help you identify the DLI, PAR, and PPFD levels. In this way, you are able to precisely calculate the light output necessary for your growing requirements.
You can boost this by using light reflectors and luminaires.
Once you secure how to optimize your LED grow light system, watch how you earn a bountiful harvest.
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.
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