In short. A BESS’s power rating is what it can deliver at once. Size it from the duty: the peak you want to shave, the load step it must absorb, the critical load it must carry during bridging, or the starting demand it must support. Then check inverter current limits, because starting kVA and fault current are limited by the inverter.
Power by duty
| Duty | What sets the power rating |
|---|---|
| Peak shaving | Peak demand minus the target cap |
| Bridging to generator | Critical load that must be carried |
| Generator support | Largest step or peak above the generator’s comfortable limit |
| Islanding or black start | Total island load plus the largest start |
| Spinning-reserve replacement | Reserve required |
Worked examples (illustrative)
Peak shaving. Demand peaks at 1,200 kW and the target cap is 900 kW. Power needed is 300 kW, plus margin and allowance for inverter and auxiliary losses, so perhaps 350 kW.
Bridging. The critical load is 400 kW for a generator start of 30 seconds. Power needed is 400 kW, plus margin. Energy is small.
Generator support. A 1 MVA generator should not take more than a 400 kW step. The site has a 700 kW step. The BESS covers the difference, 300 kW, for the first seconds.
Check apparent power, not only kW
Inverters are rated in kVA. At power factor 0.9, a 500 kVA inverter delivers 450 kW. For loads with reactive demand, the kVA limit may govern. Also check reactive power capability and whether the BESS must supply kVAr.
Starting current
Inverters limit current, often to about 1.1 to 1.5 times rated for short periods. A motor starting DOL draws five to seven times its rated current, so a BESS cannot supply it alone unless it is much larger than the motor, or the motor is soft started. See starting methods.
Fault current
Inverter fault current is limited, so protection that relied on high fault current may not trip. Check protection when the BESS is the main source. See inverter fault-current behaviour.
Other checks
- Transformer and cable ratings at the BESS point of connection.
- Round-trip losses and auxiliary loads such as cooling.
- Grid code and utility limits on export or ramp rate.
- Derating at high temperature.
Linking power and energy
Power and energy are chosen together. The duty also needs a duration. See BESS energy and BESS for critical power.
What sets the BESS power rating
Power (kW or MW) is the rate the battery can deliver or absorb. It is set by the largest event the BESS must handle, not by its stored energy.
| Use | Power requirement |
|---|---|
| Ride-through of a critical load | Critical load kW plus margin |
| Peak shaving | Peak above the target demand limit |
| Generator step-load support | Step size not carried by the generator |
| Motor starting support | Inrush kVA, noting inverter current limits |
| Frequency regulation | Contracted response band |
Worked example (illustrative)
Peak shaving: demand peaks at 1,200 kW, target limit 1,000 kW. Required discharge power = 200 kW plus 10 percent margin = 220 kW. For ride-through of a 300 kW critical load, power must be at least 300 ÷ inverter efficiency (0.97) ≈ 310 kW. Take the larger requirement, say 320 kW, and check the inverter’s kVA rating against reactive needs.
Inverter limits
- Continuous rating in kVA, which covers both kW and kVAr.
- Overload rating for short periods.
- Fault current limit, typically 1.1 to 2 times rated.
Check against generators
If the BESS supports a generator, its power must cover the shortfall during the generator’s response, with the energy sized separately. See BESS energy guide and BESS for critical power.
Common mistakes
- Sizing power on running load and ignoring starts.
- Using kW without checking kVA limit.
- Assuming BESS fault current is similar to a generator.
What to send us
Send the load profile, duty and motor list. Send your application details.
Questions engineers ask
What is the difference between MW and MWh?
MW is power, how fast energy is delivered. MWh is energy, the total delivered.
Can a BESS start a large motor?
Only within its current limit. Large DOL starts need soft starters or other support.
Does the BESS need reactive power capability?
For loads with low power factor or voltage support, yes. Check the inverter’s kVA rating.
Why does temperature matter?
Inverters and batteries derate at high temperature.
Figures are illustrative. Final sizing needs a detailed study and manufacturer data.