Calculators

Home Battery Size Calculator for Backup Power

Work out how many kWh of battery storage and how many kW of power you need to back up your home during an outage, with example battery setups.

Updated

Check your bill for kWh per day. Backing up essentials only uses far less than the whole home.

Add up what runs at the same time. Motors (AC, well pumps) need extra surge power.

Defaults come from our data files (last checked October 24, 2026).

Your estimate

Usable capacity needed

11.8 kWh

Continuous power needed
5 kW

Example configurations (typical quoted prices before incentives)

  • 1 × Tesla Powerwall 313.5 kWh · ~$13,055
  • 1 × FranklinWH aPower 215 kWh · ~$18,480
  • 3 × Enphase IQ Battery 5P15 kWh · ~$21,435

To run 10 kWh/day for 24 hours you need about 11.8 kWh of usable storage and at least 5 kW of continuous output.

This is an estimate, not a quote. Real results depend on your roof, shading, utility tariff, equipment and installer pricing.

How to fill in each input

  • Daily energy use. Take the kWh from your electricity bill and divide by the days in the billing period. The U.S. average is 28.4 kWh/day[src] (EIA, 2024). If you only want to back up essentials, start from about 4.4 kWh a day: that is what the main refrigerator, lighting, the main TV and a microwave use in the average U.S. home, according to the EIA’s Residential Energy Consumption Survey.
  • Hours of backup. How long your typical outage lasts, or the longest one you want to ride through without solar recharging.
  • Peak power. The total power of everything that could run at the same time. Air conditioners and well pumps need extra power for a moment when they start.
  • Reserve. Energy the battery keeps and does not deliver. The default of 10% matches Tesla’s note that Powerwall goes into standby when less than 10% of its energy remains.
  • Round-trip efficiency. The share of energy you get back out of the battery. The default, 89%[src], is Tesla’s rating for Powerwall 3.

Typical results

Battery capacity needed for common scenarios (10% reserve, 89% efficiency, no solar)
ScenarioDaily useCapacity needed
Essentials for 24 hours4.4 kWh5.1 kWh
Essentials for 3 days4.4 kWh15.4 kWh
Average U.S. home for 24 hours28.4 kWh/day33.4 kWh
Daily use from EIA (2024 average and RECS 2020 appliance data). Capacity is the calculator's result for each case.

For comparison, the median battery in U.S. residential solar-plus-storage systems installed in 2025 held 13.5 kWh[src], according to Berkeley Lab.

How we calculate this

  1. Energy for the outage. Daily use × hours of backup ÷ 24.
  2. Losses. We divide by the square root of the round-trip efficiency to approximate the energy lost while the battery discharges.
  3. Reserve. We divide by (1 − reserve) so the energy you need is available on top of the reserve the battery keeps.
  4. Power. The battery’s continuous output must cover your peak load, so the kW you enter is the minimum.
  5. Example setups. For each battery in our data file we take the larger of (kWh needed ÷ usable kWh per unit) and (kW needed ÷ continuous kW per unit), rounded up. Specs come from each manufacturer’s datasheet; prices are typical installed quotes before incentives from EnergySage (first half of 2026). The market average was $1,130/kWh[src], but your quote depends on the installer, electrical upgrades and how many units you add.

What the result does not include

  • Surge power. Motors (AC compressors, well pumps) need several times their running power for a moment at start-up. Check the battery’s peak or locked-rotor rating.
  • Solar recharging. With solar panels, the battery can recharge during a multi-day outage, so you may need less storage.
  • Seasons. Annual averages understate summer air conditioning and winter heating. Use a bill from the season you are planning for.
  • Cold weather. Many batteries limit charging or output in very low temperatures.

Read our home battery guides, or estimate the panels to recharge it with the solar savings calculator.

Frequently asked questions

What size battery do I need to run my whole house?

At the U.S. average of about 28 kWh a day, one full day of whole-home backup takes roughly 33 kWh of storage once you allow for a 10% reserve and discharge losses, which is about three 13.5 to 15 kWh batteries. Homes with electric heat, an electric water heater or heavy air conditioning need more.

What is the difference between kWh and kW?

kWh (energy) is how much the battery stores, which decides how long it lasts. kW (power) is how much it can deliver at once, which decides what you can run at the same time. You need enough of both.

Why does the calculator keep a 10% reserve by default?

Home batteries do not deliver their last few percent. Tesla, for example, says Powerwall stops powering the home when less than 10% of its energy remains. Set the reserve higher if you also want energy left for the start of the next outage.

Where do I find my daily kWh?

Your electricity bill lists the kWh used in the billing period. Divide it by the number of days in that period. Many utilities also show daily or hourly use in their online account or app.

Sources

  1. Electric Sales, Revenue, and Average Price 2024 (Table 5A), U.S. Energy Information Administration (accessed October 24, 2026)
  2. Residential Energy Consumption Survey 2020, end-use consumption tables CE5.3a and CE5.3b, U.S. Energy Information Administration (accessed October 24, 2026)
  3. Best Practices During Power Outages (standby below 10% energy), Tesla Support (accessed October 24, 2026)
  4. Powerwall 3 Datasheet (North America, 2024), Tesla (accessed October 24, 2026)
  5. Energy storage cost (marketplace quotes, first half of 2026), EnergySage (accessed October 24, 2026)
  6. U.S. Distributed Solar and Storage, 2026 Data Update (Tracking the Sun), Lawrence Berkeley National Laboratory (accessed October 24, 2026)