85% at least seems fairly reasonable. Your main losses will be in the motor/generator, and that sort of roundtrip efficiency is doable there. Hydro will be a bi

85% at least seems fairly reasonable. Your main losses will be in the motor/generator, and that sort of roundtrip efficiency is doable there. Hydro will be a bit worse as you'll also suffer from turbine losses, but the coupling between a motor and a concrete block will be nearly 100% efficient.