IIUC there is a physical limit from thermodynamics in converting back, based on the temperature. So when efficiencies are quoted, it could be either the proport

IIUC there is a physical limit from thermodynamics in converting back, based on the temperature. So when efficiencies are quoted, it could be either the proportion of input energy retrieved, or the proportion of the theoretical max efficiency achieved. I'm guessing you're quoting the former?