It is usually measured in watts (W). The energy storage capacity of a storage system, E, is the maximum amount of energy that it can store and release. It is often measured in watt-hours (Wh). A bathtub, for example, is a storage system for water. Its “power” would be the maximum rate at which the spigot and drain can let water flow in and out.
The power of a storage system, P, is the rate at which energy flows through it, in or out. It is usually measured in watts (W). The energy storage capacity of a storage system, E, is the maximum amount of energy that it can store and release. It is often measured in watt-hours (Wh). A bathtub, for example, is a storage system for water.
As of the end of 2022, the total nameplate power capacity of operational utility-scale battery energy storage systems (BESSs) in the United States was 8,842 MW and the total energy capacity was 11,105 MWh. Most of the BESS power capacity that was operational in 2022 was installed after 2014, and about 4,807 MW was installed in 2022 alone.
An energy storage system (ESS) for electricity generation uses electricity (or some other energy source, such as solar-thermal energy) to charge an energy storage system or device, which is discharged to supply (generate) electricity when needed at desired levels and quality. ESSs provide a variety of services to support electric power grids.
Specifically, dividing the capacity by the power tells us the duration, d, of filling or emptying: d = E/P. Thus, a system with an energy storage capacity of 1,000 Wh and power of 100 W will empty or fill in 10 hours, while a storage system with the same capacity but a power of 10,000 W will empty or fill in six minutes.
The length of time an ESS can supply electricity varies by energy storage project and type. Energy storage systems with short durations supply energy for just a few minutes, while diurnal energy storage supplies energy for hours.