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Battery Capacity (Ah) Calculator

Work out battery capacity (ah) instantly with clear inputs, formula shown and shareable results.

About Battery Capacity (Ah) Calculator

Required capacity is load current times hours, divided by the usable depth of discharge and again by an ageing and temperature derating. Depth of discharge is the big factor: a lead-acid bank limited to 50% DoD needs twice the nameplate capacity of the raw amp-hour demand.

Formula

Battery sizing

Ah = load x hours / DoD / (1 - derating)

Worked example

With the default values below, Battery Capacity (Ah) Calculator returns Battery capacity required of 27.78. Change any input above and every figure here is recalculated in your browser.

Inputs used

Load current
2.5 A
Required runtime
8 h
Usable depth of discharge
80 %
Temperature and age derating
10 %

Results

Battery capacity required
27.78 Ah
Energy at 12 V
333.33 Wh
Usable capacity from that battery
22.22 Ah
Capacity before derating
20.00 Ah

Step by step

  1. 1Values used
  2. 2Battery sizing
  3. 3Battery capacity required= 27.78 Ah
  4. 4Energy at 12 V= 333.33 Wh
  5. 5Usable capacity from that battery= 22.22 Ah
  6. 6Capacity before derating= 20.00 Ah

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

What depth of discharge should I use?

Around 50% for flooded lead-acid to get reasonable cycle life, 80% for AGM in light service, and 80-90% for lithium iron phosphate.

Why derate for temperature?

Available capacity drops roughly 20% at 0 degrees C and much more below that, while high temperatures shorten life.

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