A battery energy storage system (BESS) is a set of rechargeable batteries, usually lithium-ion cells in shipping-container-sized units, connected to the grid through inverters and a transformer. It charges when electricity is cheap and plentiful and discharges when it is scarce.
MW versus MWh
Every battery has two sizes, and they are often confused.
| Measure | What it tells you | Example |
|---|---|---|
| Power (MW) | How fast the battery can charge or discharge | 50 MW |
| Energy (MWh) | How much electricity it can hold | 100 MWh |
| Duration (MWh ÷ MW) | How long it can run at full power | 2 hours |
A 50 MW / 100 MWh battery can supply 50 MW for two hours, or 25 MW for four. Headline figures in news reports are usually in MW, which says nothing about how long the battery lasts.
How batteries earn money
Grid batteries make money in several ways: buying power cheaply and selling it at peak times, responding within a second to keep the grid’s frequency stable, being paid through the Capacity Market to be available in tight periods, and helping the National Energy System Operator manage bottlenecks on the network.
Britain’s fleet
The government’s Clean Power 2030 Action Plan put installed battery capacity at 4.5 GW and set a range of 23–27 GW for 2030. The Renewable Energy Planning Database, which tracks projects of 150 kW and above, listed about 5 GW of batteries as operational and nearly 10 GW under construction in mid-2026. Its counts lag behind commissioning, so the real fleet is likely somewhat larger.
Why it matters
Wind and solar output does not follow demand. Batteries shift that power by a few hours, cover the evening peak and can absorb power that would otherwise be curtailed. Grid batteries typically hold only a few hours of energy, so they cannot cover a week of calm, cold weather. That role needs longer-duration storage such as pumped hydro.
Read more: The environmental impact of batteries: mining, carbon and recycling.