Utility-scale batteries operated with an average round-trip efficiency of 82% during 2019, the most recent full-year Power Plant Operations Report by the US Energy Information Administration.

Pumped-storage hydroelectric projects —sometimes called “water batteries" — had an average round-trip efficiency of 79%. The round-trip efficiency is a measure of the energy lost by converting electricity to another form of energy and then converting it back into electricity.

By contrast, making green hydrogen as a way to store renewable electricity and then turn it back into electricity has a round-trip efficiency of less than 40%. In such a process, renewable electricity is used to power an electrolyzer that separates hydrogen from oxygen in water and then the hydrogen is used to run a gas turbine to generate electricity. Such a process makes sense only if it uses electricity during periods when the wind or solar project that is the power source would have otherwise been curtailed.

The weighted-average battery duration in 2019 was 1.5 hours. The weighted average duration for pumped-storage hydro was much longer. The duration refers to how long the energy remains stored before it is retrieved from storage.