Sizing Tool

Solar Battery Size Calculator

Size a home battery from your overnight energy use, days of autonomy, depth of discharge and round-trip efficiency.

Evidence-first · every figure sourcedReviewed July 2026Our methodology

Key takeaways

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.

Daily energy usekWhAverage daily household consumption.Used overnight / off-solar%Share of use a battery would cover (evening + overnight).Days of autonomydaysDays you want to ride through. 1 for daily cycling.Depth of discharge%Usable share of rated capacity. ~90–100% for LFP.Round-trip efficiency%Charge-then-discharge efficiency. ~90% typical.

Usable capacity needed

10.0kWh

Recommended nominal

11.1kWh

Nightly load covered

10.0kWh

Daily charge energy

11.1kWh

How it works

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.

Good to know

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.

Keep calculating

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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.

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.