How it works
A BESS is built from lithium-ion cells grouped into modules and racks, usually housed in containers. A battery management system watches each cell's voltage, temperature and current, and limits charging or discharging to keep them within safe ranges. The cells store direct current. A power conversion system, or inverter, turns it into alternating current for the grid and back again. A transformer steps the voltage up to the connection level. Cooling and fire suppression sit inside each container.
A battery has two ratings. Power, in MW, is how fast it can charge or discharge. Energy, in MWh, is how much it holds. A 50 MW, 200 MWh system is a four-hour battery. An energy management system sends the setpoints, deciding when to charge, when to discharge and at what power from prices, the schedule and grid frequency. Each full charge and discharge is a cycle, and cycling wears the cells. Usable capacity fades over the years, so most projects plan to add or replace modules.
Why it matters
A battery earns from timing. It buys low and sells high, firms renewable output, or holds capacity back for frequency response. The same hardware can return well or poorly depending on how it is dispatched. Duration sets which revenues it can reach. Degradation sets how long it can keep earning them. Size, duration and operating strategy have to be decided together, because each changes the value of the others.
A worked example
A 50 MW, 200 MWh battery draws 200 MWh at midday, when power costs $30/MWh, for $6,000. At 88% round-trip efficiency it returns 200 × 0.88 = 176 MWh in the evening peak at $110/MWh, earning 176 × 110 = $19,360. The day's margin is 19,360 − 6,000 = $13,360. Over 300 such days that is about $4.0 million a year, before degradation, auxiliary power and operating costs. Discharging 176 MWh at 50 MW takes about 3.5 hours.
Illustrative figures.
Across markets
In the US, batteries earn from energy arbitrage, ancillary services and, in several regions, capacity payments, and standalone storage now qualifies for the investment tax credit. In Europe, early projects relied on frequency response, and revenue has shifted towards trading as those markets filled up. In India, most batteries are procured through government tenders paying a fixed monthly charge per MW, with viability gap funding behind early capacity. Merchant revenue there is still thin.