Energy storage typically consumes electricity and saves it in some manner, then hands it back to the grid. The ratio of energy put in (in MWh) to energy retrieved from storage (in MWh) is the round trip efficiency (also called AC/AC efficiency), expressed in percents (%).

It is obviously a critical factor in the usefulness of an storage technology. The higher the round trip efficiency, the less energy we lose due to storage, the more efficient the system as whole. grid systems engineers would like to see 80% round trip efficiency in energy storage systems when at all possible.

For some use cases, these numbers do not quite reflect reality. Some technologies will experience leakage in the case of long term storage, while others will keep quasi-constant efficiency in the same circumstances. Batteries (current leakage), flywheels (friction) and electrothermal storage (heat loss) experience significant leakage over long durations, while hydro (water leaks, evaporation) and compressed air (air leaks) are quite stable.