Lighting LED Watts per Sqft

Look at the output not the power draw. Top shelf LEDs will get you ~200 lumen/W these days if you can have one of those you can go with 20W/sqft. I get my QB288v2 Rspec to deliver 2.25 gram/W with 90W in a 2x2.

@shendomoto Welcome to AFN First of all you will need about 4ft² for each plant. Watts per square foot is Bro-science and has nothing to do with the spectrum and PPFD provided to the plant. The DLI (daylight integral) is the number of PPFD presented to the plant over a day. This is the science you want to look at. Cannabis performs best with 28 - 32 DLI as a minimum and will utilize up to 60 DLI but there is a diminishing return on the light over 50 DLI. You want your lights to be full spectrum. That used to mean PAR (photosynthetic active radiation) 400nm to 700nm wave length light but new science is extending that from 350nm to 750nm.



The green in the middle has now been proven to be used by the plant as well as ultra-violet 350nm to 400nm as well as far red 700nm to 750.

Most commercial lights do not include the ultra violet (or enough) but can be added with other sources later. Many lights do now include the far red.

This spectrum is Blue heavy and will be better for the vegetative cycle:


While this leans red heavy and is better for Flowering.


Many lights allow for different spectrum by turning on or off or otherwise adjusting the spectrum.

Reputable Light manufacturers will have this information for you. You may need to do a little math to compare fixtures but here is what I feel you should look for in PPFD.

The PPFD (photosynthetic photon flux density a measure of the photons) foot print can be a shortcut to visually find a good light once the spectrum is correct. They look like this. It gives you the PPFD for a given area at a given height. You are looking for ~1000 PPFD in the center and as much as you can get on the outsides. 1000PPFD will produce about 32 DLI in 18 hours.



Two fixtures with close spectrum and PPFD can have drastic differences (efficiency) in the watts used to produce the same light. This is why watts per square foot is Bro-science.

Are you looking to buy a light?

shendomoto said:

I'm confused about LED Watts per sqft. I got this info here from: https://www.ilovegrowingmarijuana.com/5-tips-buying-led-lights/
It says that 128 watts are enough for 2 plants in 2x2 sqft, but I've seen people recommending from 500 to 1000 watts.
Am I calculating this wrong?

1′ x 1′: 32 watts
2′ x 2′: 128 watts
3′ x 3′: 288 watts
4′ x 4′: 512 watts
5′ x 5′: 800 watts
6′ x 6′: 1152 watts

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Just as explained above, when deciding on a light, the best data to consider is PAR/PPFD and spectrum, not Watts.

But, If you can't get this data, which shouldn't really be the case as most grow light manufacturers include it now a days in one form or another, and if you want to base your decision on Watts/power draw, I think the ILGM website numbers could be a better-than-nothing reference IF you are talking about lights such as HLG's, Autocob's, Mars Hydro's and other's which use good quality LEDs (recent generation from Samsung, Cree, Osram, Bridgelux, etc) or which at least have a good reputation. I believe those ILGM numbers might not be enough for old or cheap LEDs; addionaly they might even be a bit above necesary as Wico's comment can attest too if you have a quality light. I also have a board similar to Wico's (samsung diodes) for a 2x2 and find I'm pretty happy using it at just 60Watts of its 120Watts though i know I'm missing out on light penetration by using it this low and I'm sure 90Watts for my lamp would be more than enough for me.

Not sure who would recommend 500 Watts or more of LED draw power for a 2x2... not even with a blurple LEDs or screw-in LED bulbs would i think that is necessary.

Hope you got things cleared up by now.

Over the years, I've gone from hps to blurples and now to autocobs and qbs. My flower tent is at 46 watts (from wall) sq ft in a 2x3 tent and I'm getting fatter/thicker nuggs. Those older blurples are way off on their effectiveness since 500w is usually less than 100w from wall....I still run hps and blurples in a second flower tent at around 70w sq ft that doesn't come close to the cob/qb nuggs.

Loads of good information. It seems the basic starting point light with good reviews is the marshydro ts1000.
if you're looking for decent but entry level.

automan2 said:

My flower tent is at 46 watts (from wall) sq ft in a 2x3 tent and I'm getting fatter/thicker nuggs...

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I'm too scared to try going higher because of that... I have very high humidity where i grow so i'd rather have not too thick nugs to avoid rot while i'm able to lower RH here.

elcoloan said:

I'm too scared to try going higher because of that... I have very high humidity where i grow so i'd rather have not too thick nugs to avoid rot while i'm able to lower RH here.

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I solve high rh issues/rot with adding an ac/dehumidifyer to my tent for summer.

automan2 said:

I solve high rh issues/rot with adding an ac/dehumidifyer to my tent for summer.

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Thanks! i tried a dehum in the tent and didn't work so i'm saving for a big one for the room where the tent's in. In the mean time, my buds still come out nice and i get enough for my needs, plants just dont fill up as much as i now they would with more light.

By the way... damn... 70w sq ft of hps+blurples not matching what almost half of that of cobs/qbs can... @shendomoto, that shows you how watts can be misleading!