gram per watt is a great measurement. Each different wattage of light produces a different amount of lumens (we all know this). And what does light do for plants? Photosynthesis, which is breaking the light down into food for the plant. So the bigger the wattage of the light, the more food to be processed for the plant. This is where the efficiency comes into play. You can have two grows with the same lights and if one produces more, it was more efficient with the power. It was able to uptake nutrients at a faster rate. We all strive for optimum growth. Like when using c02, you can let the room temp get up to 85-90 degrees no proplem. The plants start breathing faster and as long as the c02 supplementation is there an increased growth rate occurs. This is why people use hydro too. Anything that you can do to advance the growth rate. No matter what you only have 2 months to bud that plant so you need maximum growth eficiency during that time.
An example, I have 4 plants under 3 600w lights. You must cover every square inch of your canopy. I top/bend/tie. Each plants canopy is a minimum of 2x2 (4sqft). There are an average of 30-40 cola buds per plant. Avg cola bud 7-10grams. So at least .5lbs to .75lbs comes from just the tip buds that I take out, I leave the lower buds for 4-5 days with the light still going and get another couple oz per plant.
But no matter how they are measuring it, it all come down to plant growth. How fast can it grow during the flower phase. I wouldnt use the watt hours as time can be different for every grow. You need baslines, variables that do not change...watt...square meter, each grow can have different hours becasue of genetics, but you cannot change a watt or meter.
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