From a fellow grower: Questions I have about wattage. I know a running rule of thumb is about 1g/watt. So, If for instance your yield states 18 oz/m2, is that only for outdoor? What would it be under indoor conditions and do you think it’s best to measure light by wattage or ppfd? What area should a single LED light cover?
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Depends on what light your using… for instance my lil sf1000 is only good for a 2×2 area
Usually, by the m sq means indoor coverage under quality lights.
Outdoor being sun grown is gauged by per plant yield.
Depends on the size / wattage of the light.
General rule of thumb is 50 w / sq ft for Hps / Mh.
Quality newer led’s usually 35-40 w/sq ft.
So if you wanted to flower a 4 x 4 area you’d need roughly a 400-450 w light.
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I’ve been growing for years and just bought my first led (hlg 350r), and since it is dimmable I learned about ppfd and dli. I’m a firm believer now in measuring, and setting light to the optimal range for both of these per plant stage. Not only is it great for the plants, but saves money on electricity since I don’t need to run the light on full blast all the time. I use the photone app.
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Yield estimates are pretty much BS. Plants in outdoor and indoor optimal conditions can produce huge amounts of bud. All sorts of variables are involved with light being only one of them. Most personnel use growers try to focus on quality not quantity. If quantity goes high it’s a nice bonus. But most don’t have the room indoor to push for shear quantity. Lights for horticulture have always been calculated by watts per square foot. The watts in question are what the light draws from the power source or the receptacle. NOT the marketing BS of equivalent to a 1000w or 600w HPS/MH. In the old days LPS/HPS/MH/CMH it was 45 to 50 wpsf. With HO fluorescents coming in about the same. Then came the for crap pos china burples that millions flocked to Amazon and there local grow stores to be ripped off to get. Closely followed by the better COB’s. The rule of thumb for this generation of lights was 30 wpsf. With good COB’s being good at 35 wpsf. But heat was still a issue because of low efficiency’s. With the advent of newer LED tech the min. is now basically 40 wpsf. It’s been a silent and slow progression. And it does cost $$$$ to play. So to find out what you have or what you need knowing the goal. You divide the square foot of your enclosure/tent/room into the max watt draw of the light in question. Enclosure height plays no part in the math. Only the square feet. So a common tent size now days for many is a 4’ x 4’ tent. A popular, great and expensive light for a 4’ x 4’ tent would be a HLG Scorpion Diablo @ 650 watts max draw. 4’ x 4’ tent is 16 square feet. Divide 16 into 650 for 40.6 wpsf. 40.6 wpsf is down right perfect! People seem to go way out of there way to make things far more complex than needed. Good Luck!
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Great historical breakdown @Blastfact. I used to run 1000w and 400w HPS back in the late '80’s, and when I got back into indoor, I picked up up one of those COB lights. I like it, does ok in my 4x4, but only 360w. I’d really like to put it in something more like a 3x3 tent and get a 480w LED system for my 4x4, but convincing the wife to let me get a second area and spending $650 on a light might take some time! No way she’d let me spend $930 on a 600w light (but I’ll try)!
Still, I got over 12oz on my last run with it!
Edit: These are Canadian dollars, so about $520 and $750 US respectively.
This. 
Yield estimates assume ideal growing conditions (DLI, pH, PPM, watering practices, proper grow media,…) which the average grower has a hard time replicating.
You’re going to get frustrated if your target is estimated yield from some random grow equipment seller.
@Oldguy’s metrics are on target re: watts.
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on thephotone app what is good reading for 6 day old seedling 2 illuminance or lux 1stimer
