About the PCS Sizing
Power Conversion System (PCS) sizing determines the rated power (kW/MW) of the battery inverter or bidirectional converter for a BESS — ensuring it can handle the required charge and discharge power rates without overloading, while matching the battery's C-rate limits.
Formula
PCS rated power (kW) = Battery capacity (kWh) × C-rate
- Battery capacity (kWh)
- Usable energy capacity of the battery pack
- C-rate
- Charge or discharge rate (1C = full charge/discharge in 1 hour)
How to calculate it
- 1Define the required charge/discharge duration: 1-hour (1C), 2-hour (0.5C), or 4-hour (0.25C) BESS.
- 2PCS power = battery capacity × C-rate. A 1 MWh, 2-hour BESS needs 500 kW PCS.
- 3Add derating for temperature and altitude: at 40°C ambient, most PCS units derate to 85–90% of rated power.
- 4Verify PCS matches battery's maximum C-rate: LFP cells typically support 1C charge and 2C discharge maximum; PCS must not command higher rates.
- 5For multi-string BESS: each battery rack has its own DC/DC converter; the AC/DC PCS aggregates across all racks.
Worked example
A 2 MWh BESS for 4-hour discharge (0.5C): PCS rating = 2,000 kWh × 0.5C = 1,000 kW. Derate for 45°C ambient: 1,000 × 0.87 = 870 kW effective. To maintain 1 MW effective output in summer, select a 1.15 MW rated PCS. Verify battery C-rate: 1,000 kW ÷ 2,000 kWh = 0.5C — within LFP's 1C continuous rating.
What you can calculate
- PCS kVA/MW
- IEEE 1547
- Overload 110%
- Efficiency curve
Frequently asked questions
What is the difference between a solar inverter and a BESS PCS?
A solar inverter is unidirectional — it converts DC from panels to AC for the grid. A BESS PCS is bidirectional — it converts AC to DC to charge the battery, and DC to AC to discharge. Some modern hybrid inverters combine both functions, handling both solar input and battery charge/discharge. For large BESS (> 500 kW), dedicated four-quadrant PCS units are used, offering full reactive power control in both charge and discharge modes.
What C-rate is typical for Indian grid-scale BESS?
Most grid-scale BESS in India (SECI tenders, state utility tenders) are specified as 4-hour systems (0.25C), providing 4 hours of full-power discharge matching the evening peak demand window (5–9 PM). C&I arbitrage BESS are typically 2-hour (0.5C) systems, balancing cost and ToU window coverage. Frequency regulation applications use 1C (1-hour) systems for faster response.
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