In short. A battery’s nameplate energy is not all usable. Depth of discharge, reserve, round-trip losses, temperature and degradation reduce it. Size the nameplate energy so the usable energy, at end of life, covers the duty for the runtime you need.
From duty to energy
Nameplate kWh = Energy required ÷ (DoD × η × Degradation factor × Temperature factor)
Where DoD is the usable depth of discharge, and η the inverter and system efficiency.
Worked example (illustrative)
A site wants to carry a 300 kW critical load for 30 minutes during an outage. Energy required = 300 × 0.5 = 150 kWh. Assumptions: usable DoD 0.9, system efficiency 0.94, end-of-life capacity 0.8 of nameplate and temperature factor 1.0.
Nameplate = 150 ÷ (0.9 × 0.94 × 0.8 × 1.0) = 150 ÷ 0.677 ≈ 222 kWh. A 250 kWh system would be a practical choice. The usable energy at start of life would be higher, giving extra runtime early on.
Reserve state of charge
If the BESS also serves savings or other duties, keep a protected reserve for outages. For example, if the reserve for resilience is 30 minutes of the critical load, the BESS is never discharged below that for other purposes. Size energy for both duties plus reserve.
C-rate and duration
The C-rate is the discharge power divided by energy. A 500 kW, 250 kWh battery discharges at 2C and lasts about 30 minutes at full power. High C-rate duties need chemistries and cooling that support them, and may have different efficiency and life. A longer-duration system, such as 500 kW for 2 hours, has a lower C-rate.
Degradation
- Capacity falls with cycles, calendar age and temperature.
- Warranties often state capacity retention at years or throughput.
- Design to the end-of-warranty capacity unless augmentation is planned.
Duration beyond minutes
Energy cost scales with duration. For multi-hour or multi-day outages, generators with fuel storage are usually more economical, with the BESS providing bridging and support. See DG, UPS and BESS hybrid and hybrid screening.
Other checks
- Auxiliary load for cooling and controls during the outage.
- Recharge time and source.
- Space, weight, and fire-safety design.
Energy sizing
Nameplate energy (kWh) = load kW × duration (h) ÷ (usable depth of discharge × round-trip or discharge efficiency × ageing factor)
Worked example (illustrative)
Support 250 kW for 2 hours: 500 kWh at the load. Efficiency 0.95, usable window 80 percent, end-of-life factor 0.85 (the battery retains 85 percent of capacity at end of design life, so divide by 0.85): 500 ÷ (0.95 × 0.80 × 0.85) ≈ 774 kWh nameplate. Round to 800 kWh. Power rating should be at least 250 kW ÷ 0.97 ≈ 260 kW, so a C-rate of about 0.33 per hour.
| Design factor | Notes |
|---|---|
| Depth of discharge | Limit for life, commonly 80 to 90 percent for lithium |
| Temperature | Cooling needed; capacity falls in cold, life shortens in heat |
| Degradation | Capacity declines over years and cycles |
| Reserve | Keep a floor for resilience if the BESS also does peak shaving |
| Recharge | Time and power available to recharge after use |
Duration versus economics
Doubling the duration doubles the energy cost but not the inverter cost. Choose the duration that the real outage profile and generator start time justify. Many sites need minutes, not hours, if a generator is reliable.
Check manufacturer warranty terms on throughput and temperature. See BESS control.
Common mistakes
- Treating nameplate kWh as usable.
- No reserve for resilience.
- Ignoring degradation over the project life.
What to send us
Send the load profile, duty and outage duration. Send your application details. See also BESS power.
Questions engineers ask
How much of a battery’s nameplate energy is usable?
It depends on chemistry, settings and age. Often 80 to 95 per cent at start of life, falling over time.
What is round-trip efficiency?
The energy out divided by the energy in over a full charge and discharge cycle.
Does a longer duration cost more?
Yes, energy capacity is the main cost driver.
Can I size to start-of-life capacity?
Only if you plan augmentation. Otherwise design for end of life.
Figures are illustrative. Final sizing needs manufacturer data.