Battery calculators · FREE TOOL

Battery Capacity Calculator

Estimate nominal battery bank capacity in Wh and Ah from an energy target or a power-and-duration requirement.

1

Your inputs

Edit the example
Core inputs
W
h
V

Use the nominal rating on the battery, not its charging voltage.

Planning assumptions
%

Editable planning assumption. Follow the battery maker’s limits.

%

An assumption, not a measured equipment specification.

%

Adds this percentage to the energy estimate; not a state-of-charge cutoff.

×

For example, 2 means two repetitions of the entire entered energy requirement.

Example estimate · editable inputs

Required nominal capacity

1,111.11 Wh

At your nominal voltage
92.59 Ah
Total load energy
800 Wh

Includes your usable fraction, conversion efficiency and additional reserve. Output-current and equipment limits need separate checks.

BEHIND THE RESULT

How the calculation works

Nominal Wh = load Wh × autonomy multiplier × (1 + additional reserve) ÷ (usable fraction × efficiency). Nominal Ah = nominal Wh ÷ nominal V.

A capacity estimate is one part of planning. A battery bank also has current limits, protection and equipment constraints that this calculation does not assess.

A worked example

100 W for 8 hours needs 800 load Wh. At 12 V, 80% usable, 90% efficiency and no extra reserve, the required nominal capacity is 1,111.11 Wh, or 92.59 Ah.

What this estimate can’t tell you

No battery wiring arrangements, BMS compatibility, off-grid system design or installation certification are provided.

Sources & methodology

Formulas are derived from the units shown. Read our methodology or report a correction. Sources reviewed 2026-11-01.