Battery backup calculator
Most battery sizing advice starts from your monthly bill, which overstates what you need during an outage. Start from the appliances you actually want running, for how long, and whether solar can recharge the battery.
What do you need during an outage?
Example wattages: check the label on your own appliance and edit. Hours are per day of outage.
Outage and battery
Many owners keep some charge back for the next outage. 0% uses the full battery.
Your backup estimate
- Energy needed per day
- 3.2 kWh
- Energy needed for the outage
- 3.2 kWh
- Energy the batteries must supply
- 3.2 kWh
- Peak load if everything runs at once
- 1.4 kW
- Batteries needed for energy
- 1 unit
- Batteries needed for power
- 1 unit
- Recommended: Tesla Powerwall 3
- 1 unit
- Runtime on battery alone
- 102 hours
- Estimated installed cost
- $13,055
Estimates only. With solar, at least half a day of use is assumed to be stored for the night. Cost uses EnergySage first-half 2026 average quotes per kWh for each brand; no federal credit applies to batteries bought in 2026.
Motor loads (pumps, AC, fridge compressors) draw several times their running watts for a moment when they start. Check the battery's start rating (LRA) against your largest motor.
Defaults and sources
- Appliance energy method
- Daily kWh = watts × hours used ÷ 1,000; use the wattage on your appliance label. A refrigerator runs at full wattage about a third of the time it is plugged in.U.S. DOE Energy Saver, Estimating Appliance and Home Electronic Energy Use · checked Oct 27, 2026
- Tesla Powerwall 3
- 13.5 kWh AC, 11.5 kW continuous, starts loads up to 185 LRATesla Powerwall 3 datasheet (North America) · checked Oct 27, 2026
- FranklinWH aPower 2
- 15 kWh AC, 10 kW continuous (15 kW peak for 10 s), 185 A LRAFranklinWH aPower 2 datasheet (V2.0) · checked Oct 27, 2026
- Enphase IQ Battery 5P
- 5.0 kWh usable, 3.84 kVA continuous (7.68 kVA for 3 s)Enphase IQ Battery 5P datasheet (DSH-00010-2.0, July 2023) · checked Oct 27, 2026
- Installed home battery price
- $15,647 for a typical 13.5 kWh battery (about $1,159/kWh); top brands $967-$1,429/kWhEnergySage, solar battery cost (first half of 2026 quotes) · checked Oct 27, 2026
- Solar production per kW
- Modelled for one major city per state when you pick a statePVWatts v8 (National Laboratory of the Rockies, formerly NREL): fixed roof mount, 20 degree tilt, south-facing, 14% system losses, NSRDB typical-year weather (2020; 2023 for Alaska) · checked Oct 27, 2026
- Federal credit for buying solar or a battery (25D)
- None for systems installed after December 31, 2025IRS FAQs on Public Law 119-21 · checked Oct 27, 2026
How the calculator works
Each appliance's daily energy is its watts times the hours it runs, divided by 1,000. The calculator adds the appliances you tick, multiplies by the outage length, and subtracts what your solar can put back each day at the share you set.
It then sizes batteries two ways: enough stored energy for the outage after your reserve, and enough power for the peak load if everything you ticked runs at once. The recommendation is whichever needs more batteries.
Battery backup: common questions
How many batteries do I need to power my house during an outage?
It depends on what you run, not on your whole-home bill. A fridge, lights, internet, phone charging and a microwave use about 3.2 kWh a day at the example wattages, so one 13.5 kWh battery covers roughly four days. Adding central air or an electric water heater can need two to four batteries for a single day.
How long will a Powerwall last in a power outage?
Divide its 13.5 kWh by your hourly use. Essentials averaging about 130 W last around 100 hours; a house running central air at 3.5 kW lasts under 4 hours. Solar recharging during the day can extend that indefinitely in sunny weather.
Why does the calculator check power as well as energy?
Energy (kWh) is how long a battery lasts; power (kW) is how much it can run at once. An Enphase IQ Battery 5P supplies 3.84 kVA continuously, so a 3.5 kW air conditioner plus a microwave needs at least two units even if the energy would fit in one.
Is there a tax credit for a backup battery in 2026?
Not for batteries you buy. The 30% residential clean energy credit does not apply to batteries installed after December 31, 2025. Some states and utilities offer storage rebates or pay you to join a virtual power plant.
Can I run my well pump or central AC on a battery?
Often yes, but check the start rating. Tesla lists Powerwall 3 as starting loads up to 185 LRA and FranklinWH lists 185 A LRA for aPower 2. Compare that with the LRA on your compressor or pump label, or ask your installer about a soft starter.