Of late the question on "grams per cubic ft" per KWH , per cycle , per yr , has come about with 50 watts per sq ft minimum on a horizontal grow equating to 20 watts per sq ft sideways with bare bulbs on shelves of plants on 4 walls....
AFAIK I was the first to suggest that here at ICmag in a big thread a while ago; I proposed using grams/ft^3/Kilowatt-hour
You're welcome
But, using watt per square foot is as big of a fail now, as has it always been. Time to evolve your understating dear ol' DHF.