Free calculator

Home battery sizing calculator: size the outage you actually need

Decide what must stay on, how long it must run, and how much usable energy that plan needs. You get the comparison number first, plus a clear warning when inverter power still needs proof.

1. Define the backup windowSize the outage, not the sales package

Start with how long your essentials need to run. Reserve adds breathing room. Solar refill is optional and uncertain, so leave it at zero unless you have a defensible outage-day estimate.

1 to 168 hours

0 to 70 percent held back

0 to 200 kWh per outage day

Manufacturer-published energy capacity only

2. Choose essential loadsWhat actually needs to stay on?

Edit kWh/day only if you know your measured use. The defaults are planning estimates, not appliance guarantees.

0.1 to 100 kWh/day

0.1 to 100 kWh/day

0.1 to 100 kWh/day

0.1 to 100 kWh/day

0.1 to 100 kWh/day

0.1 to 100 kWh/day

0.1 to 100 kWh/day

0.1 to 100 kWh/day

How to use the result

Treat the calculator as a buyer screen, not an electrical design. The important number is usable kWh for the loads you actually need during an outage. A good quote should name those loads and show whether the design is whole-home backup, essential-load backup, or something smaller.

If the energy target is under a few kWh, read the portable power station guide before spending on a permanent system. For an installed system, move to the quote benchmark calculator after the installer documents both energy and power.

Reviewed

How this sizing screen works

The screen adds the daily energy for selected loads, multiplies it by outage hours divided by 24, and subtracts the same outage-day share of expected solar refill. It floors net load at zero, then divides by the usable share left after your reserve. The result is rounded only for display.

This is an energy-capacity starting point, not a product-count recommendation or electrical design. Daily kWh values are editable planning estimates. Replace them with measured use when you have it. Solar refill remains uncertain because weather, season, array output, and outage controls can all change it.

The supported planning range is 0.1 to 100 kWh/day per selected load, 1 to 168 outage hours, 0 to 70 percent reserve, and 0 to 200 kWh of solar refill per outage day. The screen stops instead of publishing a target above 250 kWh.

Before choosing hardware, require a model-specific continuous and surge/start-power check for the inverter, battery, and every backed-up load.

Primary sources for the capacity comparison

The selector uses manufacturer-published capacity, not a promise of runtime or compatible product count. The source wording differs by model, so each basis is named below.