Home Battery Size Calculator
Work out how many home batteries cover your home for the number of days you choose, and check that they can carry the load.
Sample numbers only, to show how it works. Replace them with yours.
Enter how many days the battery has to last.
This is an estimate, not a professional calculation. See the disclaimer and the methodology.
How it is calculated
Energy needed = energy used per day × share to cover ÷ 100 × days of backup. Number of batteries = energy needed ÷ usable energy of one battery, rounded up.
The daily use starts at the 2024 average monthly use EIA reports for your state, multiplied by 12 and divided by 365. It is an average of all residential customers, so replace it with your own, which is your monthly kWh on the bill divided by about 30.
Use the usable energy of the battery from its spec sheet, not its total capacity. A battery cannot be fully emptied, and the usable figure already accounts for that.
The share lets you size for essentials only. A lower share needs fewer batteries but means you must choose which loads stay on, and you need a way to keep the rest off.
Power is a second limit. If you enter the highest power the covered loads draw together and the continuous power of one battery, the calculator also counts the batteries needed to carry that power, and uses the larger of the two numbers.
The result ignores inverter limits, conversion losses, the way the system is wired and whether the battery is recharged during the outage. Real designs need an installer.
Worked examples
The whole home for a day
EIA reports 863 kWh a month for the average U.S. home in 2024, about 28.4 kWh a day. To cover all of it for one day with batteries of 10 kWh usable (assumed), you need 28.4 kWh, which is 3 batteries.
Essentials for two days
Cover 40 percent of that use (assumed) for two days: 22.7 kWh, 3 batteries of 10 kWh.
When power, not energy, decides
Cover 20 percent for one day and the energy needs only 1 battery. If the covered loads peak at 12 kW and one battery delivers 5 kW continuously (both assumed), carrying the peak needs 3, so the answer is 3.
Frequently asked questions
How many kWh of battery do I need?
Multiply the kWh you use per day by the share you want to cover and by the days it must last. The calculator does that and tells you how many batteries of your size that is.
What is usable capacity?
It is the energy you can actually take out of the battery, which is less than its total capacity. Spec sheets list both. Use the usable figure.
Where do I find my daily use?
Your electric bill shows the kWh for the month. Divide by the days in the billing period. The calculator starts from the state average for a household.
Can a battery run my air conditioner or electric heating?
Those loads use a lot of power. Add their peak power to the peak field and check it against the continuous power of one battery. Many homes cover only essentials.
Does this include solar?
No. With solar the battery can recharge during the day, which lowers the days it has to last. This calculator sizes for the days you enter with no recharge.
Related calculators
Sources and last update
- U.S. Energy Information Administration, 2024 Average Monthly Bill, Residential (average monthly use by state)
- U.S. Energy Information Administration, copyrights and reuse
Last updated: November 3, 2026. See methodology and sources.