SOLAR
Solar Battery Backup: How Big a Battery Do You Need?
Size a home solar battery for backup — with the free battery calculator.
A home battery keeps essential loads running through an outage and lets you store midday solar for the evening peak. The size you need depends on how many kWh you use per day and how many days of autonomy you want.
THE SHORT ANSWER
The short answer
Most homes need 10–25 kWh of usable backup — one or two Powerwall-class units. Essentials only (fridge, lights, internet) run on 5–10 kWh. Full-house backup on an all-electric home can reach 30+ kWh.
How to size it
List the loads you want to back up, add their daily kWh, then multiply by days of autonomy and add a 20% margin for depth-of-discharge limits. A fridge, furnace fan, lights and internet typically use 10–20 kWh per day.
- Essentials only: 5–10 kWh
- Typical whole-home: 15–25 kWh
- All-electric home: 25–40 kWh
- Rule of thumb: add 20% capacity margin
What it costs
Installed lithium battery storage runs $950–$1,300 per usable kWh in 2026, so a 13.5 kWh unit lands around $13,000–$16,500 installed. Adding storage to new solar is usually cheaper than retrofitting later.
Try the battery calculator
Enter your daily backup usage, days of autonomy and cost per kWh to get a recommended capacity and estimate.