Is this calculator free to use?

Yes. The battery size calculator is free and needs no sign-up. It runs entirely in your browser.

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.

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.

Sources listedReviewed July 2026Our methodology

Inputs

View result

Daily energy useAverage daily household consumption.

Used overnight / off-solarShare of use a battery would cover (evening + overnight).

Days of autonomyDays you want to ride through. 1 for daily cycling.

Depth of dischargeUse the documented usable share for the selected product.

Round-trip efficiencyUse the documented charge-then-discharge efficiency.

Live result

Edit inputs

Usable capacity needed

10.0kWh

Indicative nominal

11.1kWh

Nightly load covered

10.0kWh

Daily charge energy

11.1kWh

Save or file this result

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.

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.
Indicative nominal divides that by the entered depth of discharge. When the documented usable share is below 100%, the nominal capacity is larger than the usable capacity required.
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.

Use the selected product's documented usable capacity, depth of discharge, efficiency and operating limits. This energy check does not select a product or verify its power and backup behaviour.

Estimates only: for guidance, not a quote or a compliance certificate. Verify against the current AS/NZS standards and a licensed design. The maths runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately save the result.

Source ledger2 sources

RegulatorBatteries — Australian Government Solar Consumer Guide
Standards bodyAS/NZS 5139:2019 — Standards Australia catalogue

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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?: The calculation runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately select Save result.
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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