Solar Battery Size Calculator

Size for the loads you need during an outage, not your whole-house use. The inverter must also handle your peak power and motor starting.

Important: Battery and inverter installations must follow NEC Article 706/480 and the manufacturer’s instructions and usually require permits.

Tapeline — Solar Battery Size Calculator

Printed September 29, 2026

Energy needed per day of backup10 kWh
Days of backup1
Usable depth of discharge90 %
Inverter / round-trip efficiency90 %

Results update as you type.

kWh
%
%

How it’s calculated

  • Usable = daily kWh × days ÷ efficiency
  • Nominal = usable ÷ depth of discharge

Worked example

10 kWh per day for 1 day, 90% depth of discharge, 90% efficiency.

Inputs

Energy needed per day of backup
10 kWh
Days of backup
1
Usable depth of discharge
90 %
Inverter / round-trip efficiency
90 %

Results

Battery capacity (nominal)
12.3 kWh
Usable energy required
11.1 kWh

Load this example into the calculator

Assumptions and limits

  • No solar recharge during the backup period (conservative).
  • Battery capacity loss with age and cold temperatures is not included.

Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.

Questions

How big a battery do I need to run my house?

List the essential loads, add up their daily kWh, and size for the days of backup you want. Whole-home backup for an average US home typically needs 25–30 kWh per day.

What is depth of discharge?

The share of rated capacity you can use without harming the battery. Many lithium systems allow about 90%.