I mean, the literal meaning of efficiency would be the capacity factor (how much energy is actually extracted out of the inputs over how much energy the input contains) which is very measurable. I'll concede that that is unfair, but even then there's the extremely accepted measurement of land use per unit of energy:
(Solar is represented as CSP, concentrated, and PV, photovoltaic)
Edit: energy over land use→land use per unit of energy