Home battery sizer
Choose what must stay on during an outage and for how long. Get the usable capacity, the power rating and a rough cost range.
What must stay on?
Your battery size
Capacity includes ~10% for depth-of-discharge and conversion losses. Solar recharge assumes 4 sun hours × 80% per day. Peak kW assumes listed loads could run together; motor start-up surges can be 2–3× higher. Cost range uses $1,000–$1,400 per kWh installed as a broad planning figure — get quotes.
How to size a home battery
Two numbers matter. Energy (kWh) is how much the battery stores — how long it can run your loads. Power (kW) is how much it can deliver at once — whether it can start your well pump and run the fridge at the same time. A battery that's big on kWh but small on kW can still trip when a heavy load kicks on.
Worked example
Fridge (1.5 kWh/day) + lights (0.5) + Wi-Fi and devices (0.6) = 2.6 kWh/day. For one day of backup: 2.6 ÷ 0.9 ≈ 2.9 kWh. Add a sump pump and freezer and you're near 5.5 kWh — still within a single typical 10–15 kWh battery. Add central air and you jump to 30+ kWh, which is why whole-home backup costs so much more than essential-loads backup.
Solar changes the picture
With panels, the battery can recharge each sunny day, so a multi-day outage needs far less storage. The "with solar" figure assumes your array refills part of the battery daily. In winter or long storms, plan conservatively.
Essentials panel vs whole-home
Most installers either move critical circuits to a "backup loads panel" (cheaper, smaller battery) or back up the whole home with load management. Read the home battery guide for the trade-offs.
Frequently asked questions
How big a battery do I need for my house?
For essentials only, many homes need 10–15 kWh. Backing up heating, cooling or an electric water heater for a day can require 30 kWh or more.
Can a battery run my air conditioner?
Yes if the battery's continuous and surge power ratings cover the AC's start-up current, but it will drain capacity quickly. A soft-start device can reduce surge.
Is a battery worth it without solar?
For pure backup, maybe. For savings, batteries without solar mainly pay off where time-of-use price gaps are large or where utilities pay for grid services.