Nominal energy is calculated from the capacity and the average voltage across the discharge:
E (Wh) = C (Ah) × V_avg (V)
In practice, cell datasheets often quote nominal energy using the nominal voltage rather than the true average discharge voltage, which gives a slightly simplified figure. For accurate energy calculations — particularly in pack sizing — integrating voltage over the full discharge curve gives a more precise result.
Why nominal energy differs from usable energy
The nominal energy figure represents the full discharge from 100% to 0% state of charge under standard conditions. In real applications, the battery management system restricts operation to a defined state of charge window to protect cell longevity, so the usable energy delivered to the system is always less than the nominal cell energy. A pack sized to 100 kWh nominal might deliver 90 to 95 kWh usable depending on the SoC window set by the BMS.
Scaling from cell to pack
At pack level, nominal energy is the sum of all cell energies accounting for series and parallel configuration. Losses from wiring resistance, contactors, and thermal management overhead mean pack-level usable energy is always lower than the simple sum of cell nominal energies. For pack sizing methodology, see Pack Sizing.
Nominal energy as a benchmarking metric
Nominal energy is one of the primary metrics used to compare cells across chemistries and formats. Dividing by cell mass gives gravimetric energy density (Wh/kg); dividing by cell volume gives volumetric energy density (Wh/L). Both figures are condition-dependent in the same way as nominal energy itself — quoted values on datasheets assume the same standard test conditions. See Key Cell Metrics and Nominal Capacity.
See also: Nominal Voltage, Cell Gravimetric Energy Density, Usable Energy