What's the most honest way to express grams per watt?

I think grams/watt is really an arbitrary measurement given all of the other factors in the life and health of the plant. Veg time introduces the largest variable but other factors like co2 concentrations, foliar feeding, strategic defoliation, variations in temperature and RH as well as growing technique all affect final yields. Unless some scientific research critera is applied where the variables are standardized the grams/watt idea, in my view comes up lacking as a reliable measurement.

It depends on your goals. If you are trying to compare different lights, g/dissipation W is very useful. If all the variables are the same except the light used in flowering, you have a reasonably objective comparison.

Or another way it can be useful is for comparing two different genotypes. If all the variables are the same except the geno, then you can objectively say which one yields better.

g/kWh is more useful for determining the profitability of your grow or comparing your overall efficiency.

Who really cares. so far as i've always been aware, gpw is pretty pointless, and is generally used for nothing but to see if you are pulling a suitable yield for the light you have. Nothing to do with the wattage of fans and pumps and such, simply if you yield say 0.2gpw, none of the other variables matter, you're doing a shit job, and if you are yielding 1gpw then you can just take this as you doing a good job for the light you are using. It means absolutely bugger all in terms of comparisson.

I think everyone agrees that gpw is pretty well useless, other than a rough and easy calc that requires no history/work
g/kwhr can be useful if you take the time to log/measure everything relevant, but requires a lot of discipline to be useful
g/kwhr may be used for improving efficiency (cost, yield for a particular strain) , but could also be useful for testing new lights (only if comparing the same strain grown with previous lights), or comparing training/grow methods for the same strain with the same equipment
Key for g/kwhr or any measurement is comparing same strain grows, or possibly deciding on what strains to grow if your driver is yield
If you're constantly flipping strains, there is no real benefit in measuring anything for the sake of comparison, only useful data would be to measure a baseline against a new strain you plan to continue growing imo

gpw is dumb, treat your grow like a business and measure your return on investment, how much does it cost me to produce 1 gram. That should include everything from power from start to finish to grow medium and nutes. If you want to use the entire AN line then add it up, if you brew teas add it up, if you use new soil every grow add it up, if you reuse your medium theres nothing to add after the first grow. If you pop seeds under a 23w cfl add it up, if you run a cloner add it up. At the end of the day ROI is all that matters. Equipment is another thing, the more you spend on equipment, the longer it will be before you get a return. If you run a bare bulb and buy a new hort every grow add it up, if you have $2000 in led add it up. when the equipment is paid for there is nothing to add, upgrades, repairs, bulbs, filters etc all get added. When you do it that way you can sit down and figure out exactly how profitable you are and figure out where you can improve.