Home Battery Size Calculator: What Size Do You Need?
How many kWh of battery storage do you need? Our free home battery size calculator estimates your backup need, units required, and 2026 installed costs.
How big a home battery do you actually need?
A home battery size calculator takes the guesswork out of one of the most expensive decisions in home energy: how much storage to buy. Oversize and you waste tens of thousands of dollars; undersize and your system dies halfway through the outage you bought it for. This free tool estimates your backup need in kilowatt-hours (kWh) based on your home's size, whether you want critical loads or whole-home coverage, and how many hours of outage you want to ride through.
How the estimate works
We start from typical daily electricity use for your home size — roughly 25 kWh per day for a smaller home up to 80 kWh for a 6,000-square-foot estate. Critical-loads backup typically needs about 35% of daily use; whole-home backup needs it all. We scale that by your target outage hours, add 20 kWh if you plan to charge an EV during outages, then round up to whole 13.5 kWh battery units — the most common residential module size. Installed costs in the 2026 US market typically run $900 to $1,300 per kWh of storage, including equipment, labor, panel work, and permits.
Frequently asked questions
It depends on your home's daily use and how long you want coverage. A 2,500 sq ft home uses roughly 35 kWh per day, so 12 hours of whole-home backup needs about 17.5 kWh — roughly two 13.5 kWh units once you round up. Larger homes with pools, multiple HVAC zones, or EV charging can need 40+ kWh. The calculator above walks through your specific scenario.
Critical loads are the circuits you choose to keep running during an outage — typically refrigeration, Wi-Fi, lighting, security, medical equipment, and sometimes one HVAC zone. It usually requires about 35% of the storage a whole-home setup needs, which is why it costs dramatically less. Most battery installers recommend starting here unless your budget supports full coverage.
A large luxury home (4,000–6,000 sq ft) targeting whole-home backup through a 12-hour outage typically needs three to five 13.5 kWh units, or 40–68 kWh of nominal storage. Homes with heavy loads like pool heaters or multiple EVs trend toward the high end. A licensed installer should confirm with a load study before you buy.
Yes, if the outage happens during daylight or spans multiple days. Solar panels recharge your batteries each day, so a smaller bank can cover a multi-day outage that a battery-only system could not. For single overnight outages, though, solar does not help — your evening consumption comes entirely from stored energy.
Installed costs typically range from $900 to $1,300 per kWh of storage in the US market, so a 27 kWh system (two units) generally lands between $24,000 and $35,000 before any incentives. Panel upgrades, long wire runs, and complex permitting can push costs higher. Always get itemized local quotes from licensed installers.