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I dont agree with their assessment
A 1000w HID has always been advertised as the industry standard for covering a 4 x 4 area. 62.5w Sq/ft. Thats been the standard since 1978, when HID for horticulture first came out.
A 600w HPS/Hortilux HPS is the most efficient SE 600w HPS I know of, and is advertised as covering a 3 x 3 area. Thats 66w sq/ft. 40w Sq/ft is way to low, especially for equatoral sativas where the light index is off the charts. Especially in Columbia, and Equador, and in the Andes mountains. UVA/B is also off the charts.
Along the coast of Equador the UVA/B index is over 20, and can approach 26 at noon on a sunny day where normally in USA the UV index goes to 10
At 20,000 feet in Equador the UV index has exceeded 40!!!! Though nothing grows there. But it shows you how powerful the light is.
Now HID lights have gotten more efficient, and have Single Ended HPS with added blue. This didnt happen until the 90s, and before that people usually used a Metal Halide, or combined MH/HPS.
The AVERAGE clear sky sun provides around 893 Watts/m2(which is about 283.6
Btu/ft2/hr or very close to
100 Watts per square foot). Notice that this number is an AVERAGE value.
At the equator it can approach 1400w Sq/m. Even more at high altitude on a clear day.
A Gavita 1700e is also advertised as covering a 4 x 4 area at 645w, and 1781 umol. A 1000w Hortilux HPS is 1690umol after a 1 month break in. SE HID loses 10% the first month. At 645w the Gavita 1700e is 40.3w Sq/ft, and 1781umol , of course it less umol at 18-24 inches as is the HID.
Thing is about using all this power is that you either have to have CO2 enrichment, or constant huge fresh air exchange. I agree with their assessment if CO2 enrichment, or fresh air flow is not optimal.
But the sun can be 126w Sq/ft at the equator on a sunny day.
Ive used a 1000w HID for seedlings since 1977-78. I figure they get blasted outside/equator on a clear day, and I give then no less inside.