BESS

Battery Energy Storage System (BESS) is the industry term for a stationary battery installation, including the battery packs, the inverter (power conversion system), the battery management system, and the housing.

A BESS can be sized from tens of kilowatt-hours (small commercial) to hundreds of megawatt-hours (utility-scale). It can sit behind the meter or in front of the meter. It can be used purely for backup, purely for market participation, or (most commonly) for both.

A UPS is a specific type of BESS that's optimised for the backup use case. Not every BESS is a UPS, but every UPS is a BESS.