Backup planning

Battery backup calculator for outages and essential loads

Use this battery backup calculator to list what should stay on during an outage, how many of each appliance you have, and how long each one needs to run. The calculator keeps power, energy, and surge estimates separate.

Build the outage load list

Quick add Adds one editable example

Appliance Watts Qty Hours Surge W

Load previewbefore battery and inverter losses

Daily energy: --
Running power: --
Listed surge total: --

These are editable examples, not fixed appliance ratings. Replace them with measured or nameplate data.

System voltage VNominal battery system voltage.

Battery chemistry preset

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

Battery module for a countOptional

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

Module voltage V Module capacity Ah

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

load Wh = watts × quantity × hours; battery design Wh uses the shared battery-sizing formula

Each appliance row becomes watt-hours, then the row totals are sent to the same loss-adjusted battery sizing function used by the solar battery calculator. Running watts and entered surge watts remain separate outputs.

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: essential home loads

Run a 120W refrigerator for 8 hours, a 15W router for 8 hours, and four 9W lights for 5 hours.

Refrigerator: 120 × 1 × 8 = 960 Wh.
Router: 15 × 1 × 8 = 120 Wh.
Lights: 9 × 4 × 5 = 180 Wh.
Total delivered energy: 1,260 Wh before battery and inverter losses.

Result: The appliance list needs 1.26 kWh delivered. The final nominal bank is larger after the selected DoD, efficiencies, and reserve are applied.

Common mistakes to avoid

Adding watts and watt-hours as if they are the same unit.
Multiplying surge power by the full backup duration.
Using a generic refrigerator wattage without checking its duty cycle or startup behavior.
Sizing the battery but ignoring the inverter's continuous and startup limits.

Questions people ask

Is this a home battery backup calculator?

Yes, for a first-pass outage energy estimate. List only essential loads, use measured or documented wattage, and replace the examples with the duration you need to cover.

Does surge wattage change battery kWh?

Not in this first-release model. Surge is shown as a separate power check because short startup events do not equal hours of continuous energy use.

Why does each row have its own hours?

During an outage, a router may run continuously while lights or a laptop run for only part of the period.

Can this choose an inverter?

No. It reports running and listed surge totals, but final inverter selection needs verified surge duration, power factor, waveform, and manufacturer limits.

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