Battery Atlas

Sizing & runtime

Battery Capacity Calculator (Ah)

Find the nominal amp-hour capacity you need to run a load for a given time, accounting for usable depth of discharge and inverter/conversion losses.

Power in W/kW, or current in A.

V

%

Typical: 50% lead-acid, 80–90% LiFePO4.

%

Use ~85–95% with an inverter, 100% for direct DC loads.

Formula

Wh = P × t ÷ η
Ah(usable) = Wh ÷ V
Ah(nominal) = Ah(usable) ÷ DoD

Worked example

100 W for 5 h on 12 V, 50% DoD, 90% efficiency → 555.6 Wh → 46.3 Ah usable → 92.6 Ah nominal.

Limitations and warnings

  • Rated Ah is normally specified at the 20-hour rate (C20). Higher currents deliver less capacity (Peukert effect) in lead-acid batteries.
  • Cold temperatures and ageing reduce available capacity; add a margin.
  • This sizes energy storage only. It does not determine starting (CCA) requirements.

Four values that are often confused

Nominal capacity (Ah)
Charge a battery delivers under rated conditions, usually over 20 hours (C20) at 25 °C.
Usable capacity
The part of the nominal capacity you can use without damaging the battery — about 50% for lead-acid, 80–90% for LiFePO4.
Cranking current (CCA)
Current deliverable for a short time in the cold to start an engine. It measures power, not stored charge, and cannot be converted to Ah.
Stored energy (Wh)
Ah × voltage. The right unit to compare batteries of different voltages.