12V Install Tools & Testers

Daily Energy Use & Battery Size Calculator

List what you'll run each day, check the assumptions, and get an estimate of the battery capacity you need.

This gives an energy-use and battery-capacity estimate only, not a complete system design. Add each item you'll run in a typical day. The example rows are only a starting point, so edit or remove them. Everything is worked out in your browser. Nothing is saved or sent anywhere.

1. Your loads

Enter the power in watts (check the label or manual), the hours it runs per day, and how many you have. Choose AC (via inverter) for 240V AC appliances powered through an inverter. For items that cycle on and off, like a compressor fridge, use the average running hours or the maker's daily energy figure.

2. Assumptions

These defaults are common starting points, not rules. Change any of them to suit your gear.

These are nominal system voltages. Actual battery voltage varies with chemistry and operating conditions; for example, a nominal 12V LiFePO4 battery is commonly around 12.8V.
Choosing a chemistry sets a typical usable fraction below. Typical starting value only. Check your battery manufacturer's recommended depth of discharge and operating limits.
How much of the rated capacity you plan to use (10 to 100%).
Days the battery must last with no charging (0.1 to 30).
Extra allowance for ageing, cold and estimates (0 to 100%).
Applied to AC loads only (50 to 100%).

3. Results

How it was worked out

    Before you buy a battery

    • This calculator provides an energy-use and battery-capacity estimate only. It is not a guaranteed battery specification, and it is not a complete system design, wiring specification or safety assessment.
    • Actual sizing also depends on the battery maker's limits (maximum discharge current and recommended depth of discharge), temperature, discharge rates, surge and start-up loads, inverter standby draw, cable losses, your charging arrangements and other system requirements. Check the manuals for your battery, inverter and appliances.
    • Inverter standby (no-load) power is not included automatically. If your inverter stays on, add its standby watts as a DC load running 24 hours.
    • Solar and charging are a separate step. This version does not size solar panels or chargers. Take your daily energy use to our Solar & Charging Check next. Your charging sources need to put back at least your daily use, with some margin. See What Size DC-DC Charger Do I Need? and Solar Blanket vs Rigid Solar Panel.
    • Any work on 240V AC wiring, including inverter AC outputs and mains inlets, must be done by a licensed electrician.

    Next steps

    Use your result with the Calculate & Size Your System section of our Build Your System guide, then compare battery types in Lithium vs AGM Battery for Camping.

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