a gram per watt - not so sure that is how it works but i could be wrong - i do not have any major fancy lights - i do have a 600 (198 led's ) watt full spectrum that does run about 140 to 160 actual watts and a red or blue spectrum helper light at about 19 watts ( 60 led's )
even though my personal best was about 193 grams - this did not include the other lower sections that i trimmed off twice and those were about 53 to 60 grams each time so take about 120 extra grams to the 193 = 313 grams and that is a good overall total harvest for 1 plant in a 3.5 X 3.5 chamber without having top high end lights or equipment
to me its more about training the plant to get the maximum coverage to the footprint of the light at both the closest possible before burning and the maximum height where the total area has about even light in that footprint - while trying not to lose total par of the light and what the plant needs during the grow
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Depends on genetics, environment and training. I always hear you should get a gram per watt of light, but I rarely see it..