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Great question!
Tangent time - a 6-8 week flowering cycle on an auto is more like a 12-16 week flowering cycle on a photo, when taking into consideration light-hours and total watts used (assuming the auto is flowering in the 18-24/day range). In that respect, autos are terribly inefficient!
Back to your questions -
Watts per sq/ft is generally calculated excluding the duration component. Total watts / total sf. And it's generally sf of the grow space, not the area occupied by the plant.
For example -
I'm currently running 200w in an 8sf space, or an equivalent 25w/sf.
If I was to factor in the duration component, in this case an 18hr cycle, it would look like this - (18/24)*(200/8) = 18.75w/sf. This is the true measure of consumption averaged over a daily cycle.
Yield per sf is calculated generally the same - total yield / total sf.
If I hypothetically pull 300gr in my setup, I would have a yield average of 300/8 = 37.5gr/sf and an gr/w average of 300/200 = 1.5gr/w.
As you can see, it's not a fair comparison, you truly need to factor in the duration component to get a true representation of a particular setups efficacy, relative to a comparison (i.e.: your grow vs. my grow).
You bring up a good point about autos verses photos .......
When Sam and Goldie were in Flower, they were using 254 Watts of light - 200 Watts center over Sam and 54 Watts over Goldie .....
Space was 2x4 .......
Goldie looks like 100 grams dried and partially cured at 77% RH now ...... Sam the jury is out ...... but for this example let’s say 10.5 oz yield from Sam ..... which would be total for the tent of 14oz ..... so would math be:
14oz / 8 ..... equals ...... 1.75 oz per Sq Foot
392 Grams / 254 Watts ....... equals ...... 1.54 Grams Watt