Maybe space, square footage could be a proxy for time, like g/watt/sq. ft. Reason is, you could grow one big plant in a long time, but many small plants in a short time. In the end though the space is filled, doesn't matter how you got there. Then again, maybe you are on the right track with something like grams per watt-hour, where efficiency is (pot out / electricity in), which sounds attractive as a raw efficiency number. Is it really as simple as electricity = pot + heat?
Do experienced growers count their popcorn buds in the g/watt total? Been at 0.6 for a while (only counting good bud, maybe a little bit over 1 with the stuff that gets made into edibles), and not getting to that 1g/watt standard feels bad.