About the Usable Capacity
Usable capacity is the actual energy a battery storage system can deliver per cycle after applying the Depth of Discharge (DoD) limit, state-of-charge (SoC) window, and round-trip efficiency. It is always less than nameplate capacity and is the correct figure to use in energy balance and backup calculations.
Formula
Usable capacity (kWh) = Nameplate × DoD × Round-trip efficiency
- Nameplate
- Battery's rated energy storage at standard conditions (kWh)
- DoD
- Depth of discharge allowed: fraction from max SoC to min SoC (e.g. 0.80)
- Round-trip efficiency
- AC-to-AC efficiency of the full charge-discharge cycle (e.g. 0.90 for LFP)
How to calculate it
- 1Start with nameplate capacity from the battery datasheet.
- 2Apply the operational DoD: LFP systems are typically operated at 80–90% DoD; lead-acid at 50%.
- 3Multiply by round-trip efficiency (charge efficiency × discharge efficiency).
- 4The result is usable energy per cycle — use this in backup time, peak-shaving, and LCOS calculations.
Worked example
A 100 kWh LFP battery bank: DoD = 0.90, round-trip efficiency = 0.93. Usable capacity = 100 × 0.90 × 0.93 = 83.7 kWh/cycle. To store and dispatch 80 kWh net, you need at least a 100 kWh nameplate BESS.
What you can calculate
- Usable kWh
- DoD derating
- Temp derating
- Aging factor
Frequently asked questions
Why is usable capacity less than the nameplate rating?
Three losses reduce usable capacity: (1) DoD limit — operating lithium batteries to 100% SoC or 0% SoC stresses them and reduces cycle life, so a SoC window (e.g. 10–90%) is kept; (2) round-trip efficiency — energy is lost as heat during charging and discharging; (3) temperature derating — capacity falls at extreme temperatures. The nameplate is a theoretical maximum; usable capacity is what the system actually delivers.
What round-trip efficiency can I expect from an LFP BESS?
Modern LFP BESS systems achieve 90–95% round-trip efficiency (AC-to-AC) at rated power and temperature. At low power or extreme temperatures, efficiency drops. PCS (power conversion system) losses account for 3–5%, and battery internal resistance losses add another 1–3%. A conservative design assumption of 90% is appropriate for energy planning.
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