The right number of LED grow lights, or rather the power of LED grow lights, for indoor growing can roughly be estimated based on the watt per square foot (or square meter) principle.
As light output from LED grow lights somewhat correlates to consumed power (watt), planning the light setup for a grow space by using established watts per square foot recommendations is a quick and easy way to estimate one’s light needs. Here’s what you need to know.
Choose The Right LED Light For Your Grow Space By Calculating Watts Per Square Foot
In the growing community, it is not uncommon to purchase grow lights based on the “watt per foot (or square meter)” principle. This can be an easy and useful method assuming that equally good lamps are compared to one another. If PPFD maps (more on this later) aren’t presented by the lamp manufacturer, the watt per square foot principle may be the only reasonable way to assess a lamp’s ability to light up a grow space. Some lamp types perform significantly better than others (operate at higher efficacy, PPE or µmol/J) and will require fewer watts to emit equal amounts of light, so compare apples to apples and oranges to oranges. Efficacy of Photosynthetic Photon Efficacy (PPE), tells us how efficiently a lamp converts electricity (watts) into light (photons).
Two lamps that both draw 100 watts but one operates at an efficacy of PPE 1.0 µmol/J and the second lamp operates at an efficacy of PPE of 2.0 µmol/J. As the first lamp is only half as efficient, it will only produce half as much light (photons) as the second lamp. In other words, the second lamp will emit twice as much light as the first lamp but draw the same amount of power (watts).
The presumption of the watt per square footage principle is: to figure out how many grow lights one needs for a grow space, all one has to know is the space’s area, a 3x3-foot or nine square foot tent, for example. Then use the established “rule” for how much power a certain grow area needs.
Watts Per Square Foot Rule of Thumb
Low efficacy, generic “Amazon” lights (PPE: ~1.0 µmol/J): 50-60 watts per square foot (525-640 watts per square meter)
Medium efficacy, “housing”-type lights (PPE: ~1.5 µmol/J): 30-40 watts per square foot (320-430 watts per square meter)
High efficacy, panel with quality diodes (eg. Samsung, Osram), or (Cree) COB-type lights with high-end driver (PPE: ~2.5 µmol/J): 25-30 watts per square foot (265-320 watts per square meter)
To fill a 3x3-foot (nine square foot) tent using high efficacy lights, a total consumption of 25 to 30w*9 square foot = 225-270 watt is needed. This could be achieved by either one lamp or several lamps.
To fill a 2x2-foot (four square feet) tent using medium efficacy lights, a total consumption of 30 to 40w*4 = 120-160 watts is needed. This could be achieved by either one lamp or several.
These would be ideal conditions. Plants will naturally also grow under less than ideal conditions but possibly not quite to their fullest potential. Using a 200-watt lamp instead of a 225-watt lamp in a 3x3-foot space would also work very well, even though it’s slightly under the guidance. Further reducing light to 150 watts or 100 watts would, however, significantly reduce yield. On the other hand, increasing the watts per square foot above recommendations can potentially increase the yield although the increase may be small or even marginal. But there’s more to it.
(Read also: How To Set Up Your LED Grow Light)
Consider the Spread of Light
After determining roughly how strong LED grow lights are needed for a grow space, the next aspect to consider is how well the light spreads over the grow area.
Some lamps come with focusing lenses which give high intensity underneath the lamp but low intensity farther away from the center. A lamp should as evenly as possible cover the grow area with light, reaching PPFD around 600-900 µmol/m2/s but preferably no less than 200 µmol/m2/s. Light intensities below PPFD 200 drive photosynthesis poorly. Compare different lamps’ footprint with one another; which covers your grow area best?
Because high efficacy lamps emit more light per consumed watt, mainly due to their highly efficient diodes and driver, this type of lamp requires fewer watts to achieve the same light output as a simpler lamp. Diode brands like Samsung, Osram, Cree, and some Bridgelux models, can deliver twice or higher quantities of light than generic brand diodes. Good diodes accompanied by a reliable and efficient driver like Mean Well or Inventronics make for a highly efficient system.
All LED lamps are not created equal. Outer appearances can make two vastly different LED lamps look the same. But it’s the total build quality, diodes, driver, and light spread that matter.
As an example, the newly released LEDTonic Q7 quantum panel LED grow light delivers a high PPFD and also offers a good, even spread of light. With its double-panel design, it reduces otherwise common light intensity hotspot and covers a larger area with uniform light intensity. As the panels are independently horizontally and vertically adjustable, the Q7 offers great flexibility to any grow space.
It can both be used in 3x3-foot and 4x4-foot tents with great results. Check out the PPFD maps below to see how the average PPFD will vary depending on the grow area.
Here we see the LEDTonic Q7 LED grow light, with a 320-watt output, PPFD measured in both a 4x4-foot and a 3x3-foot grow tent. We see that the results in the 4x4-foot tent give an average PPFD of 496 while the smaller 3x3-foot tent has an average PPFD of 793.
Applying the watts per square foot principle results in a value of 20 watts per square foot in the 4x4-foot tent and a value of 35 watts per square foot in the 3x3-foot tent. The light footprint map clearly shows how applying the principle can give a rough estimate of how well an LED grow light illuminated a grow space, but again, looking directly at PPFD values gives a clearer picture.
An easy and quick way to estimate how powerful LED grow light is needed for a particular grow space is to measure the space’s area in square feet (or square meters) then multiply the area with proven constants, as follows:
Low efficacy, generic budget lights: 50-60 watts per square foot (525-640 watts per square meter)
Medium efficacy, “housing”-type lights: 30-40 watts per square foot (320-430 watts per square meter)
High efficacy, panel with quality diodes (eg. Samsung): 25-30 watts per square foot (265-320 watts per square meter)
Two by two feet, four square feet or 60x60 cm, 0.36 square meters of space
- Low efficacy lamp: 200-240 watts
- Medium efficacy lamp: 120-160 watts
- High efficacy lamp: 100-120 watts
Four by four feet, 16 square feet or 120x120 cm, 1.44 square meter space
- Low efficacy lamp: 800-960 watts
- Medium efficacy lamp: 480-640 watts
- High efficacy lamp: 400-480 watts
Again, these values are generalized and not written in stone. A lamp with a good light spread would require fewer watts to fully illuminate a specific area. On the other hand, a lamp with poor light spread would require more watts to illuminate the same area.
Still, applying the watts per square foot principle gives a fair idea of how powerful a lamp is required for the job.