Home battery backup calculator
Pick what has to stay on in an outage. It works out how much power that takes at once, the kick to start the motors, and the kilowatt-hours to get through, then lists the batteries that cover it and how many you'd need.
Built by Aatish Patel, founder of XCharge North America
How it works
- Running power adds up what you picked, allowing for the fact that big loads rarely all run together: everything up to 5 kW counts in full, and 60% of the rest.
- Surge is the running power plus the start-up kick of the single biggest motor, because motors rarely start in the same instant. A soft starter shrinks the kick from an air conditioner or heat pump.
- Energy is each appliance's running power times how much of the day it actually runs, for the season you pick. A fridge runs about a third of the time. An air conditioner in July, about half.
- Batteries are sized to clear all three, keeping 5% in reserve. Solar, if you add it, refills during the day. Tesla's expansion units add energy but no power, so the tool uses them where energy is the only shortfall.
Prices are typical installed figures for 2026, before incentives, and vary a lot by region and installer. Surge ratings come from the manufacturers' datasheets. Where a manufacturer doesn't publish one, the tool says so.