Home Battery Size Calculator

How this battery sizing calculation works

The calculator starts with the energy you want to keep available during an outage: essential-load kWh per day × backup days. It then divides that target by usable depth of discharge and usable efficiency. The result is the minimum nominal battery capacity required under the assumptions you entered.

The formula is nominal capacity = required delivered energy ÷ (depth of discharge × efficiency). The battery-unit result then rounds up to the next whole unit using the nominal capacity per battery. This distinction matters because batteries are sold in discrete sizes: the minimum calculated capacity can be lower than the nominal capacity you actually purchase after rounding.

Worked example using the page defaults

Assume essential loads consume 10 kWh/day and you want one day of backup. With 90% usable depth of discharge and 90% usable efficiency:

• Required delivered energy: 10 × 1 = 10.0 kWh.
• Combined usable fraction: 0.90 × 0.90 = 0.81.
• Minimum nominal capacity: 10 ÷ 0.81 = 12.35 kWh, displayed as about 12.3 kWh.
• If each battery is 13.5 kWh nominal, the calculator rounds 12.35 ÷ 13.5 up to 1 battery unit.

The displayed “effective delivered energy” is the requested 10 kWh at the calculated minimum capacity. A real 13.5 kWh unit may provide some additional margin under the same assumptions because the commercial unit is larger than the 12.35 kWh minimum. Manufacturer reserve settings and operating limits can change how much of that nominal energy is actually available.

Energy capacity is only half of backup design

Battery capacity in kWh tells you how long loads can run; inverter power in kW tells you how much load can run at once. A battery can contain enough energy for a full day and still be unable to start a large air conditioner, well pump or other high-surge load. Whole-home backup therefore requires both an energy budget and a peak/surge power check.

What can make real outage runtime shorter or longer

Actual runtime depends on the loads that turn on during the outage, battery state of charge when the outage begins, reserve settings, temperature, battery age, inverter losses and whether solar or another source recharges the battery. A “one-day” design based on average essential-load use does not guarantee 24 hours if high-power equipment runs more than expected.

The U.S. Department of Energy describes batteries as devices that accept, store and release electricity on demand, and notes that storage can support resilience when paired with appropriate system design. See DOE Explains: Batteries and the DOE Homeowner's Guide to Solar for storage context.

When to use this estimate vs. an installer design

Use this tool to turn your essential-load estimate into a first-pass kWh target and to see how backup duration, reserve depth and efficiency affect required capacity. Before purchasing equipment, verify actual load profiles, peak and surge power, critical-load-panel design, code requirements, transfer/islanding behavior and the manufacturer's usable-capacity specification.