Battery sizing

Solar Battery Bank Calculator

Size storage from daily energy use and the number of days without charging. Use this page for a battery-bank capacity target; use Battery Backup for an appliance outage list and Battery Runtime for an existing battery.

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

Enter both module values to include a whole-battery layout, or leave them blank for a capacity-only result.

Module voltage V Must divide evenly into system voltage. Module capacity Ah Rated capacity of one matching module.

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.

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.

Maintenance and verification

Maintained by: SolarMathKit Formula version: 1.3.0 Last reviewed: July 27, 2026

The calculator calls the shared, tested TypeScript calculation modules documented in the methodology. Automated tests cover published examples, invalid inputs, rounding, and result invariants.

SolarMathKit maintains this implementation. The review is an internal technical check, not an independent engineering certification.

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.

How many Ah do I need at 12V, 24V, or 48V?

Enter the same energy target and choose the nominal system voltage. The calculator converts the loss-adjusted kWh result to Ah; the same energy needs fewer amp-hours at a higher voltage.

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.

Does this calculator also size solar panels?

No. It sizes storage from an energy target. Use the solar panel calculator after the load and battery assumptions are settled, because panel sizing also depends on local solar resource and system losses.

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: Check voltage, current, temperature, protection, cable sizing and installation rules with the equipment manuals and a qualified professional.