Battery Capacity and Runtime Calculator
Calculate the battery capacity required for a load, or estimate runtime from Ah, voltage, power and visible efficiency assumptions.
How the battery capacity and runtime calculation works
The calculator uses an energy-balance model. First it adds the load power and the device's own consumption. For capacity sizing, it multiplies total power by runtime, divides by efficiency, converts watt-hours to amp-hours at the selected battery voltage, and applies the sizing factor. For runtime, the same model is reversed: available battery energy is adjusted by efficiency and divided by the sizing factor and total load. The result is a planning estimate, not a guarantee. Actual runtime also depends on battery chemistry, age, temperature, discharge rate, wiring, inverter cut-off and manufacturer limits.
Load energy (Wh) = (load W + self-consumption W) × runtime h
Battery energy (Wh) = load energy ÷ efficiency
Required capacity (Ah) = battery energy ÷ voltage × sizing factor
Estimated runtime (h) = capacity Ah × voltage × efficiency ÷ sizing factor ÷ total load W
Worked examples
12 V, 1000 W load, 6 W self-consumption, 60 min, 90% efficiency, factor 1.25: 1006 Wh at the load, 1117.78 Wh from the battery, 116.44 Ah required.
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24 V, 1400 W load, 6 W self-consumption, 40 min, 90%, factor 1.25: 937.33 Wh at the load, 1041.48 Wh from the battery, 54.24 Ah required.
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12 V, 100 Ah battery, 100 W total load, 90%, factor 1.25: 864 Wh usable in this model and about 8 h 38 min estimated runtime.
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Frequently asked questions
What is the sizing factor?
It is an explicit multiplier used by this model when selecting capacity. The default 1.25 comes from the project spreadsheet; it is not presented as a standard. Adjust it only when your design assumptions justify it.
Why is efficiency below 100%?
Converters, UPS units and wiring lose energy. Use the efficiency stated for your equipment and operating point when available.
What is self-consumption?
Power used by the inverter, UPS or other electronics in addition to the connected load.
Why does runtime divide by the sizing factor?
Runtime reverses the capacity-sizing equation. Dividing preserves the same reserve assumption instead of counting it twice.
Can I use the calculator for lead-acid and lithium batteries?
The energy equation is chemistry-neutral, but usable capacity, cut-off, temperature behaviour and allowed discharge differ. Use manufacturer data to set conservative assumptions.
Why can real runtime be shorter?
Age, cold, high discharge current, voltage drop, inverter cut-off and loads that vary over time can all reduce runtime.
Should I enter 12 V or 24 V for a series bank?
Enter the nominal voltage of the complete bank supplying the load, not the voltage of one battery.
Is this result a battery recommendation or a standard?
No. It is a transparent planning estimate based on the displayed inputs and the project calculation model.
Sources
- Product-specific inputs: battery and inverter/UPS manufacturer documentation.