Round Trip Efficiency (RTE)

Round trip efficiency (RTE) is the energy a battery returns divided by the energy put into it over a full charge and discharge cycle, measured at the AC terminals.

Losses occur in the cells, in the power conversion system in each direction, in transformers, and in the auxiliary load of cooling and controls. An AC to AC figure includes all of them and is the one that matters for a bill or a market settlement. Round trip efficiency is one of the inputs to arbitrage economics: a price spread has to exceed the losses before a cycle earns anything.