Solar Battery Size Calculator
Size a home battery from your overnight energy use, days of autonomy, depth of discharge and round-trip efficiency.
- Sizes a home battery from your overnight energy use and how many days of autonomy you want.
- Factors in depth of discharge and round-trip efficiency to separate usable from nominal capacity.
- Shows the usable and nominal kilowatt-hours and how much of your nightly load is covered.
Usable capacity needed
10.0kWh
Recommended nominal
11.1kWh
Nightly load covered
10.0kWh
Daily charge energy
11.1kWh
A battery only needs to cover the energy you use when the sun isn't — mainly the evening and overnight. We start from your daily use and the share of it that falls off-solar:
- Usable capacity needed =
daily use × overnight share × days of autonomy. - Recommended nominal divides that by depth of discharge— you can't use 100% of a battery's rated capacity, so a 90% DoD battery needs to be rated a little larger.
- Daily charge energy divides the nightly load by round-trip efficiency — the solar energy you must put in to get that much back out after charge/discharge losses.
LFP batteries typically allow ~90–100% depth of discharge and ~90% round-trip efficiency; your product's datasheet has the exact figures.
Estimates only — for guidance, not a quote or engineering specification. Nothing you enter is stored or sent anywhere; the maths runs entirely in your browser.
Frequently asked
- Is this calculator free to use?
- Yes. The battery size calculator is free and needs no sign-up. It runs entirely in your browser.
- Is my data stored or sent anywhere?
- No. The calculator runs in your browser and stores nothing. The figures you enter are not saved or sent anywhere.
- What is the difference between usable and nominal capacity?
- Nominal capacity is the battery's total rating, while usable capacity accounts for depth of discharge and round-trip efficiency. The tool estimates both from your inputs.
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