How To Calculate for LED Wattage Per Square Foot of Grow Space?
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As you probably know by now, there are various types of grow lights available in the market today. One of the most widely used is the LEDs. If you are still new to growing with this kind of light, then you have come to the right place.
In this article, we will discuss everything you need to know about LEDs. From the basic facts up to the calculation of how much wattage needed per square foot, we got it all covered here.
Contents
Factors to consider when choosing for the right LED grow light?
For first time buyers, here are some points you have to consider when choosing the right LED grow lights.
 Type of plant and its stages.
Of course, it is pretty much essential to consider the type of cultivar you intend to grow with. Plants thrive on varying levels of light intensity. Some like the peppers and tomatoes are favorable to use at high light conditions. Meanwhile, others like herbs and green leafy vegetables are best grown in low light.
Moreover, you also have to take into account the current stage of the plant’s development. In every phase, the amount of light required also differs. Thus, when buying LED bulbs you need to foresee the need to adjust from time to time as the plants grow.
 Type of LED grow light.
As mentioned earlier, the light requirements differ from one stage to another.
If you intend to grow plants from seedling up to its flowering stage, then a socalled fullcycle LED bulbs would be more appropriate for you to choose. It is convenient to use and it can also spare you some savings from buying different types of bulbs.
Our top picks for fullcycle LED bulbs:
Bloomspect 600W LED Grow Light
King Plus 1000W LED Grow Light Double Chip FullSpectrum
Spider Farm SF1000 LED Grow Light
There are also other models that offer settings that are targeted to cater to specific stages. The singlechannel LED bulb provides a simple on and off feature. Meanwhile, the twochannel models feature separate modes. They come with customizable program settings for the vegetative and the flowering stages.
Our top picks for singlechannel LED bulbs:
Higrow 36W LED Plant Grow Light Bulb 460nm Blue
Byingo LED Grow Light Bulb 50W 450460nmAllDeep Blue Spectrum
Higrow 36W All Deep Red 660nm LED Grow Light Bulb
G8LED 90W Red Flower Booster LED Grow Light
Our top picks for twochannel LED bulbs:
Growing Lamps LED Grow Light 200W
ZWD Upgraded 300W LED Grow Light with TwoChannels
Vivosun 1200W Cree Cob LED Grow Light
 Size of grow space.
Another major consideration when choosing an LED bulb is the dimensions of the growing area. Generally, an average LED grow light would draw about 32 watts to cover a square foot of grow space.
The bigger wattage in LED lighting doesn’t equate to bigger coverage. As opposed to what it usually represents in other types of light systems. These are metal halide (MH), highpressure sodium (HPS), and highintensity discharge (HID) lamps.
Furthermore, it is good to emphasize that wattage alone should not be the main determinant of area coverage. Nor does it dictate the level of effectiveness of LED lights. Rather, it’s used to provide an estimation of the correct fixture size in a given grow space.
The table below serves as your quick guide:
Size of Grow Space  Ideal Wattage  Approximate Watt Range  Our Top Picks 
1 x 1 or 1ft2  32 watts  30 to 40 watts  Azawa Grow Lights 30W 
2 x 2 or 4ft2  128 watts  120 to 140 watts  Derlights 120W LED Grow Light Bulb 
3 x 3 or 9ft2  288 watts  250 to 300 watts  Viparspectra UL Certified 300W LED Grow Light 
4 x 4 or 16ft2  512 watts  500 to 650 watts  Parfactworks 500W LED Grow Light 
5 x 5 or 25ft2  800 watts  700 to 900 watts  Apollo Horticulture GL140X5LED 700W LED 
6 x 6 or 36ft2  1,152 watts  1,000 to 1,200 watts  Sunraise 1000W LED Grow Light FullSpectrum 
Wattage is commonly used to represent the approximation of area coverage. But, Photosynthetic Photon Flux Density (PPFD) and the Daily Light Integral (DLI) are what counts as more accurate metrics. They measure the exact light intensity requirements over the size of the grow area. These will be discussed next.
Different terminologies to understand.
Before proceeding into the actual calculation of LED wattage, we must first discuss the different terms and metrics involved in LED lighting for horticultural purposes.
Photosynthetic Active Radiation (PAR)
Photosynthetic Active Radiation, or PAR, refers to the wavelength of light within the visible spectrum range of 400 to 700 nanometers. PAR itself is not a metric. Instead, it’s used to determine which type of light gets required to help photosynthesis in plants.
Photosynthetic Photon Flux (PPF)
Photosynthetic Photon Flux, or PPF, is the measurement of PAR light emitted by the light source per second. The PPF measurement is often expressed in micromoles per second or mol/s.
Take note though that PPF only pertains to the amount radiated by the light system. Thus, not the actual amount of light that will land on a specific surface area.
Photosynthetic Photon Flux Density (PPFD)
Photosynthetic Photon Flux Density, or PPFD, is the measurement of the amount of PAR light that actually reaches a specific surface area. It is often expressed in micromoles per square meter per second or mol/m2/s.
Technically, as PPFD increases, the rate of photosynthesis also escalates in effect. As a grower, you have to aim to achieve a suitable level of PPFD. This is to intensify the photosynthesis rate. Thereby, greater chances of getting more yield in the end.
Daily Light Integral (DLI)
Daily Light Integral, or DLI, refers to the cumulative measurement of the number of photons that reach the plant during the photoperiod. You can think of it as somewhat like the plant’s daily dose of light. DLI is expressed as moles per square meter per day or mol/m2/d.
It is important to identify your cultivar’s DLI so that you can program your lighting system. It helps deliver the number of lights based on its specific daytoday requirements.
How to calculate LED wattage correctly?
Now, onto the very highlight of our article. The steps on how to calculate the correct LED wattage for your grow space.
Step 1: Determine the grow space size in meter squares.
The precise measurement of the grow space is fundamental in figuring out the correct wattage. To do this, you can measure the width of the area in meters, then multiply it by the depth in meters too.
Using a 4×4 grow space size as an example,
For reference, 1 ft = 0.30 m
Width = 4 ft or equal to 1.2 m
Depth = 4 ft or equal to 1.2 m
Width X Depth = Area (m2)
1.2 X 1.2 = 1.44 m2
Thus, a 4×4 grow room is equal to a 1.44 m2 area.
Step 2: Establish your desired PAR light intensity.
PAR represents the areas of the visible light spectrum. It’s where the plant utilizes for its photosynthesis. These blue, green, and red wavelengths range from 400 to 700 nanometers. To simplify, the higher the PAR output of your LEDs, the more light gets absorbed by your cultivars.
A PPF requires at least 1,100mol/m2/s or more to cover a 4×4 grow space or an area with 16 square footage. Besides, the most appropriate amount of PAR to use still depends upon the current phase of development of the cultivar.
Below are the optimal PAR outputs at different stages of plant growth:
Stages  PAR Output 
Seeding Stage  200 to 400 mol/m2/s 
Vegetative Stage  400 to 600 mol/m2/s 
Flowering Stage  600 to 1,000 mol/m2/s 
PAR outputs can actually reach as much as 1,500 mol/m2/s if given the right atmospheric conditions. The most optimum level that can help an increase in growth rate and abundant yield is at 750 mol/m2/s.
Step 3: Determine the PPFD per watt of your LED light system.
PPFD measures the number of photosynthetic active photons that actually falls on the 1 square meter of the target area per second. So, the higher the PPFD, the greater would be the rate of photosynthesis of the plants in effect.
The table below presents the associated efficiencies or PPFD per watt. They are on most LED grow light systems available in the market today.
Type of LED Bulb  PPFD per Watt 
Budget Red/Blue  0.9 to 1.2 PPFD per watt 
High Power Red/Blue  1.3 to 1.5 PPFD per watt 
FullSpectrum LED  1.2 to 1.5 PPFD per watt 
Step #4: Compute the wattage.
Now, you already have the area in meter square, your desired PAR level, and the PPFD per watt of your LED light system. You may finally be able to compute for the wattage.
So, going back to our example in Step #1,
Given:
Area = 1.44 m2
Desired PAR level = 750 mol/m2/s
PPFD per watt = 1.4 PPFD per watt (assuming you are using a FullSpectrum LED)
To get the PPFD:
Area X desired PAR level = PPFD
1.44Ã‚Â X 750 = 1,080 PPFD
To get the wattage:
PPFD / PPFD per watt = Wattage
1,080 / 1.4 = 771 Watts
So, you would likely be needing about 771 watts to cover a 4×4 or a 16ft2 grow room size. You also have to take into consideration other elements. For instance, your target plant density and other lighting conditions. They may somehow affect your wattage requirements in actual.
What are the factors affecting the LED wattage?
Now, you already know how much wattage would likely need to cover your grow space. Let us move on to discussing the major considering factors to look into when determining the LED grow light wattage.
 The number of plants you intend to grow.
For every size of grow space, the number of suitable cultivars and its corresponding growth technique varies. Moreover, this also alters the number of lights you would need for your area.
The table below shows the techniques used. It comes along with the approximate quantities of plants it can cater to per square meter.
Grow Technique  Number of Plants 
LowStress Training (LST)  4 plants per square meter 
Sea of Green (SOG)  4 to 16 plants per square meter 
Screen of Green (ScrOG)  1 plant per square meter 
Pruning  1 plant per square meter 
Generally, it is ideal to divide 1 square foot of area per plant. Most LED grow lights though can cover between 1 to 6 plants per bulb. If you aim to grow more, then try installing various evenlyspaced lights. This is to encompass all sides of the canopy.
 The number of LED units per plant.
LED lights come with specific wattages. For instance, a 600watt LED would draw out only about 240 to 260 watt of electricity. This can achieve the same power as a 600watt HID or HPS lights.
Read Also: Best LED Grow Lights in 2020: Top 10 Reviews & Buyer’s Guide
On that note, LEDs are always known to be far more efficient than other types. Its actual electricity consumption is estimated to be only about 40% than its rated power.
In terms of plant quantity, the table below shows how LED power increases. This is in relevance to various plant densities.
No. of Plants  1 square foot/plant  2 square feet/plant  4 square feet/plant 
1  30 to 40 watts  60 to 80 watts  120 to 140 watts 
2  60 to 80 watts  120 to 140 watts  240 to 300 watts 
4  120 to 140 watts  240 to 300 watts  500 to 560 watts 
6  180 to 200 watts  360 to 400 watts  700 to 860 watts 
8  240 to 300 watts  500 to 560 watts  900 to 1,100 watts 
10  500 to 340 watts  600 to 800 watts  1,100 to 1,400 watts 
12  340 to 380 watts  700 to 900 watts  1,300 to 1,700 watts 
So, how many plants can you grow then?
As mentioned above, it is best to divide 32 watts per square footage.
In this example, let us say you have 5 plants in a 6 square feet space.
5 X 6 = total footage is at 30 square feet
30 square feet X 32 watts = 960 watts LED grow lights
Here are some more samples for different plant quantities and area coverage:
Coverage  Actual LED Watts  Units of LED Grow Lights 
2×2  128 watts 

2×3  192 watts 

2×4  256 watts 

3×3  288 watts 

4×4  512 watts 

4×6  768 watts 

5×5  800 watts 

3. The height difference between LED light and the plant canopy.
The distance of the LED from the plant canopy has a direct impact on its light intensity. You have to always maintain the right and safe height to achieve the potential maximum yield.
Placing the bulbs too close to the cultivars runs the risk of burning, bleaching, and possible nutrient deficiencies. Meanwhile, hanging it too high up also reduces its efficiency to deliver the right amount of light to the plants.
The details below show the recommended LED grow light heights over the canopy.
LED Wattage  Height/Distance 
240400 watts  16 to 30 inches 
450550 watts  20 to 30 inches 
600850 watts  24 to 26 inches 
900 watts and above  26 to 42 inches 
Final Thoughts
In conclusion, an LED lighting system draws up an average of 32 watts per square footage. There are a lot of varying factors that also come into play. They include the density of plants, the quantity of LED units, and even the height difference between the light over the canopy. Hence, it is very important to take everything into account. This will help you to arrive at the most favorable amount of light power for your cultivar’s development.
Indeed, determining the right wattage of LED grow lights can be quite overwhelming. Especially, for beginner growers. Nonetheless, once you understand how an LED lighting system works in relation to your plant’s growth, the process then becomes easier over time. Give yourself a little bit more practice and patience to learn through it all.