Battery sizing

Solar battery calculator

Turn daily or monthly energy use into the nominal battery capacity you need. The result separates delivered energy, conversion losses, reserve, and the whole number of matching battery modules.

Runs in your browserEditable assumptionsPlanning estimate

Inputs

Size the battery bank

Energy use Use an energy bill, monitor reading, or appliance total.

Days of autonomy How long the bank should cover without charging. Nominal system voltage V 12V, 24V, 48V, or a documented custom value.

Battery chemistry preset Editable starting point. Check the battery maker's operating limits.

Battery module for an integer layoutOptional

Module voltage V Must divide evenly into system voltage. Module capacity Ah Rated capacity of one matching module. Include this module in the battery count

Advanced assumptionsEditable planning values

Depth of discharge % Usable share of rated battery capacity. Battery efficiency % Energy retained after battery-side losses. Inverter efficiency % Used for AC conversion. DC runtime ignores it. Reserve margin % Extra nominal capacity above the loss-adjusted result.

Planning estimate

Your planning estimate

Local calculation

Enter your values, then calculate. Results will show the formula inputs and rounding used.

Formula used

design Wh = daily Wh × autonomy ÷ (DoD × inverter efficiency × battery efficiency) × (1 + reserve)

Daily energy is first multiplied by the number of no-charge days. The calculator then divides by each usable-energy factor, adds the reserve, and converts the result to amp-hours at the chosen nominal voltage.

See the full assumptions and rounding policy.

Worked example: 5 kWh per day for two days

Assume a 24V system, 90% depth of discharge, 92% inverter efficiency, 95% battery efficiency, and no extra reserve.

Energy to deliver: 5,000 Wh × 2 = 10,000 Wh.
Combined usable factor: 0.90 × 0.92 × 0.95 = 0.7866.
Nominal capacity: 10,000 ÷ 0.7866 = about 12,713 Wh.
Amp-hours: 12,713 ÷ 24V = about 529.7 Ah.

Result: Plan for about 12.7 kWh nominal, or 529.7 Ah at 24V, before rounding to whole battery modules.

Common mistakes to avoid

Using battery nameplate kWh as if all of it is usable.
Applying 90 as a multiplier instead of converting 90% to 0.90.
Rounding the Ah result down to fit a preferred battery count.
Assuming any module voltage can form the target system voltage.

Questions people ask

Why is the battery result larger than my daily use?

Daily use is energy delivered to loads. The battery must also cover depth-of-discharge limits and conversion losses, plus any reserve you choose.

Should I use daily or monthly energy?

Use whichever source is more reliable. Monthly energy is divided by 30 for a planning-day average, so check seasonal peaks separately.

Is the chemistry preset a specification?

No. It is an editable planning value. Use the battery maker's limits for a real design.

Before you build: This calculator is for planning. Check voltage limits, current limits, temperature corrections, protection devices, cable sizing, and installation rules against equipment manuals and a qualified professional.