Battery Bank Size Calculator

Turn your daily energy use into a rated battery size, then check the current, real module sizes and ageing.

This turns your daily energy use into a battery size, using the method in How to Size a Battery Bank for an Off-Grid System. The starting figures are that guide's small workshop example, so change them to your own. Everything is worked out in your browser. Nothing is saved or sent anywhere.

1. Your energy use

From the Daily Power Needs Calculator, including its margin for losses.
No-load draw from the inverter's specification, if it stays on all day. 0 if it's switched off or already counted.
Days of autonomy (0.5 to 14).
Everything that could run at once. Used for the battery current.

2. Your battery

Take it from the data sheet. As a rough guide, lithium (LiFePO4) is most of its rated capacity and lead-acid about half.
The rated voltage is the usual LiFePO4 figure. Use your maker's.
Used for the battery current.
The size the maker sells. 0 to skip the rounding step.
From the data sheet. 100 to skip the ageing check.

3. Results

How it was worked out

    Before you rely on it

    • Check the battery's maximum continuous discharge current, surge rating and maximum charge current against its data sheet. A battery big enough in kWh can still be the wrong battery if it can't deliver the current.
    • Batteries deliver less in the cold, and many lithium batteries won't charge below about 0°C without heating.
    • Joining batteries? Follow the maker's limits on series and parallel, and don't mix chemistries, sizes, ages or models unless the maker says so.
    • A bigger battery needs more panels to refill it. See What Size Solar System Do I Need?
    • This is a planning estimate only, not a system design, wiring specification or safety assessment. Check the data sheets and manuals for your battery, inverter and charge controller.
    • Safety: in Australia, any work on 240V AC wiring, including connecting an inverter, battery or solar system to a home's switchboard or fixed wiring, must be done by a licensed electrician. Grid-connected systems also need approval from your electricity network.

    Next steps

    Read the full method in How to Size a Battery Bank for an Off-Grid System. High battery current? Check the 12V / 24V / 48V Picker.

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