Watts Per Gallon applies roughly to T-12, and not too bad to T-8, though it is a little bit off. By the time you get to better bulbs and better reflectors it is so far off that it is not worth asking about.
Like asking how many miles you get from a bale of hay.
When horses were in use, that was a valid question.
Useless when 'horsepower' refers to a car.