A BESS is a wall of battery racks behind a Power Conversion System (PCS) that turns grid AC into DC for charging and back into AC for discharging. A Battery Management System (BMS) tracks each rack's State of Charge (SOC) and enforces safe charge/discharge limits.
Power rating: P = C-rate × E_capacity
Charging: ΔSOC = C-rate × η_charge × Δt_hours
Discharging: ΔSOC = −C-rate × Δt_hours / η_discharge
Round-trip: η_RT = η_charge × η_discharge
E.g. a 192 MWh site at 0.5C charges/discharges at 96 MW;
a full 0-100% cycle takes about 2 hours at that rate.
- Operating mode — Charge draws power from the grid into the racks; Discharge sends stored energy out to the load; Auto-cycle switches automatically at 2%/98% SOC to demonstrate arbitrage.
- C-rate — charge/discharge power as a fraction of capacity per hour; higher C-rate means faster SOC change but more heat and stress on the cells.
- Installed capacity — number of active racks; more racks means more MWh stored and more MW deliverable at the same C-rate.
- Round-trip efficiency — energy lost to heat over one full charge+discharge cycle (typically 85-95% for grid-scale Li-ion BESS).
Real installation: Hornsdale Power Reserve (Australia), 150 MW / 194 MWh, built by Tesla — it provides frequency regulation, energy arbitrage and backup reserve from the same battery wall.