Careful there, you are teetering on the edge of misuse of units. Calories, BTUs, Ergs, & Joules are measures of energy whereas horsepower and watts are measures of energy per unit time which is power either created or consumed. The difference is rather like conflating distance and velocity where both contain a spacial measurement but only one takes time into account.
As long as you are measuring gain as output Vs input and compare energy to energy or power to power the choice of units cancels out leaving "gain" a dimensionless figure of merit, that is gain IS the "base for comparison".
Regards,
GtG
PS Suggested reading assignment "Dimensional Analysis", not nearly as daunting as it might sound. Algebra only needed, no relativity required.
I was really being facetious. I started to add ‘ding-dongs?’............