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Home Battery Size Guide: How Much Capacity Do You Actually Need?
Battery capacity is about matching your evening load and backup goals — not buying the biggest unit. Here’s how to size one honestly.
WaterFuelAndSolarEnergy.com Editorial Team Updated January 2025 9 min read
A home battery is measured in usable kilowatt-hours (kWh). The usable figure — not the raw cell capacity — is what determines how long you can run your home in the evening or through an outage.
Work out your evening load
Look at your smart-meter or utility data for a typical evening (say 5pm–midnight) and total the kWh you use. That is the minimum useful capacity for shifting solar into the night.
Rough capacity guidance
| Household profile | Typical usable capacity | Notes |
|---|---|---|
| Small apartment, low evening use | 5–7 kWh | Backup for essentials |
| Average home | 10–13 kWh | Evening solar shift + basics |
| High use or whole-home backup | 15–20+ kWh | Heating, EV or outages |
Backup vs economic sizing
For backup, size to the circuits you must keep running. For saving money, size to your evening load and the tariff window. Optimise for your goal — a battery that does both is often slightly larger and costs more.
Will one battery run my whole house through a long outage?
Only with careful circuit selection. Whole-home backup typically needs multiple batteries or a generator, and a larger inverter.