Energy systems
Battery nominal and usable energy
- where
- usable energy (Wh)
- nominal battery voltage (V)
- battery capacity (Ah)
- usable fraction
Explain this equation
Estimates the usable energy stored in a battery, in watt-hours. Use it when sizing battery storage, UPS or off-grid systems.
V_nom is the battery's nominal voltage. Q is its capacity in ampere-hours, the charge it can deliver. f_usable is the fraction you can actually use without going outside the permitted depth of discharge, as a decimal. E_usable is the usable energy in Wh.
Energy is voltage times charge. Ampere-hours are a measure of charge, so V × Ah gives Wh, and multiplying by f_usable limits it to the safe operating window. The ≈ is there because the voltage changes during discharge and capacity depends on discharge rate and temperature.
A 12 V 100 Ah battery holds about 1200 Wh nominal, or about 960 Wh at 0.8 usable. Two in series give 24 V 100 Ah; two in parallel give 12 V 200 Ah; either way the energy doubles.
Watch out: Ah on its own is not energy, so always include the voltage. Real usable energy depends on discharge rate, temperature, ageing and battery management limits.
Nominal voltage multiplied by ampere-hours gives a nominal watt-hour estimate. Multiply by the stated usable fraction to estimate the DC energy available within the chosen operating window.
Cells in series increase voltage while ampere-hours stay the same; matched parallel strings increase ampere-hours. Actual usable energy depends on battery conditions and controls.
Enter your values to see the result.
Common mistake
Calling Ah an energy unit without including battery voltage.
Limits
First-order estimate. Manufacturer discharge curves, temperature, ageing, discharge rate, BMS limits and system losses determine delivered energy.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.