Solar Battery Size Calculator

How this calculator works

Usable energy needed = kWh per day × days of backup. Because batteries should not be drained completely, the total capacity is the usable energy ÷ the depth of discharge (90% by default for lithium iron phosphate batteries).

The result is converted into common home battery sizes of 10 kWh and 13.5 kWh. A whole U.S. home averages about 28 kWh a day (865 kWh a month, EIA 2024); backing up essentials only usually takes far less.

A battery also has a power limit in kW. Check that it can deliver enough power at once for your largest loads, such as a well pump or central AC — the generator size calculator helps add those up.

Example: 10 kWh of essentials for 1 day

1. Usable energy: 10 × 1 = 10 kWh.
2. Capacity at 90% usable: 10 ÷ 0.9 = 11.1 kWh → one 13.5 kWh battery or two 10 kWh batteries.

Tips before you buy

• Find your real daily use by dividing a monthly bill’s kWh by the days in the billing period.
• For long outages, pairing a battery with solar recharges it each day and cuts the capacity you need.
• Use the generator size calculator to compare the cost of backup with a generator.