Ok, I could nit pick some of your earlier points, but Im too tired. Lets just take this one point for starters.
Yes, watts is joules/second. However, KWH is joules/second/hour - plus or minus a decimal place - which is a whole different animal. You cannot get to cost from watts alone.
You have to do an additional calculation based on an additional measurement.
Thank you! You just agreed with the basic point I have been trying to make from the beginning.
Unless you are both willing and able go to extraordinary effort in terms of esoteric measurements, and complex calculations - watts all by themselves - are a useless metric to use. Which means g/watt is also useless. 
The one exception you give (other than your personal situation where you have all that esoteric info and equipment on hand), is in a personal grow situation where the watts, and the distance to the canopy, and the footprint of the light in terms of sq meters of canopy, never change.
My question then is - given the situation where the watts and all the rest never change - why bother to do the math at all? It would be much easier, and just as valid, to just use the total grams and be done with it.
I could argue your point about knowing where a grow falls on Bugbees graphs, but Ive already pointed out why no one, other than you, has any clue where there plants fall on the curve. Plus, Im too tired and it would amount to splitting cats hairs as far as anyone else is concerned. 
Doesnt matter. You cant get to cost without adding in that extra measurement of time and doing the extra math.
I can guarantee you that your electric company IS doing that math
But - I will grant you that IF your lights are on 24/7/365, and always at the exact same power draw at all times, then you could use the watts # (either one) to substitute for the cost of running your lights when you are figuring g/dollar of lighting cost.
But lets take another look at your specific case where your watt draw from the wall never changes 24/7/365 - which you never mentioned before by the way. Plus, note that very few people can make that claim.
That does indeed take the time out of the calculation - point to you.
However, I’ll ask you the same question I asked Northernloki - if the watts never ever change - why bother doing the math at all?
If your lights run 24/7/365, that means your light costs never change. Every single grow you do has the same exact cost to run the lights, so why divide grams by watts? What does that tell you that grams by themselves doesnt tell you - with less effort?
If you got 2000 grams from 1000 watts on one grow thats 2 g/watt. If you got 4000 grams on the next grow thats 4 g/watt, or exactly twice as much.
But 4000 grams / 2000 grams is still that same exact 2/1 ratio, so grams alone tells you everything you need to now without doing any math.
What usefulness does the watt number have in this case?
Based on what you said - no change 24/7/365 - then your personal watt draw from the wall doesnt affect your costs from one grow to the next either does it? The cost to run your lights never changes. Every grow you do is the same exact cost in terms of "watts"and dollars no matter what.
Im not going to accuse you of the same thing - quite
You got that right. For some strange reason I dont feel like Im making much progress bashing through this particular brick wall with my head 
Worst pain I have ever felt at times. Fortunately, its not always that bad. If your buddy is a grower, tell him kidney stones are worse than g/watt - but not by much. Not sure how much that will help though.
something for the spank bank 
maybe more cats in less space is more effective!
