Battery Energy Storage System (BESS): How It Works and What Software It Needs

A battery energy storage system (BESS) is a set of batteries, a power conversion system (PCS), a battery management system (BMS) and an energy management system (EMS) that stores electricity and releases it when it is needed. The IEA counted 108 GW of new BESS capacity in 2025, up 40%. In a BESS, software decides whether storage does its job. What is a battery energy storage system (BESS)? A battery energy storage system is a complete installation that charges from the grid or a renewable source, holds the energy in battery cells and discharges it on command. Batteries alone do not make a BESS. Without a PCS to convert current, a BMS to protect the cells and an EMS to decide when to charge and discharge, a rack of batteries cannot serve the grid or a building. The 108 GW figure for 2025, 40% more than in 2024, comes from the IEA Global Energy Review 2026. An IEA commentary from 29 May 2026 breaks it down: Around 87 GW of that capacity, roughly four-fifths, was utility-sc