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Guide · 2026-11-11

How to convert amp-hours to watt-hours

Multiply amp-hours by the battery’s nominal voltage. Watt-hours = amp-hours × volts. A 12 V battery rated 7 Ah is 84 Wh. That is the opening example on the Amp-hours tab of the UPS calculator, which does the multiplication for you.

Watt-hours are the number runtime math needs. An amp-hour figure on its own is not enough, because the same amp-hours hold different amounts of energy at different voltages.

Why the voltage matters

An amp-hour is current over time. Energy is current times voltage over time. A 7 Ah battery at 12 V and a 7 Ah battery at 24 V do not hold the same energy. The second one holds twice as much: 84 Wh against 168 Wh. Two labels that print the same Ah can only be compared when they print the same voltage. Convert both to watt-hours and the comparison is fair.

Milliamp-hours

Divide mAh by 1,000 to get amp-hours, then multiply by the voltage. Arithmetic only, not a product: 10,000 mAh at 3.7 V is 10 Ah × 3.7 V, which is 37 Wh. The same 10,000 mAh at 5 V would be 50 Wh. A mAh figure without the voltage it was rated at cannot be turned into watt-hours. If the label also prints watt-hours, use the watt-hours.

When a label prints both, use the watt-hours

Amp-hour figures are often rounded. EcoFlow’s Australia product page for the DELTA 2 prints 1024 Wh written as 85 Ah at 12 V. 85 × 12 is 1,020, not 1,024. The 85 Ah has been rounded from about 85.3. The difference is small here, but it is the reason RunTime takes the watt-hours a maker prints and does not rebuild them from amp-hours. The DELTA 2 page cites that product page.

Going the other way, amp-hours = watt-hours ÷ volts. That is how a maker can print one battery as 85 Ah at 12 V. It does not mean the station puts out 12 V at its AC outlets.

From watt-hours to hours

Watt-hours are the battery, not the runtime. Usable watt-hours are the capacity times an efficiency, and hours are usable watt-hours divided by the load in watts. On an AC outlet the efficiency is the inverter’s. Both WattWise tools default to 85%, a labeled planning assumption, and both let you change it. The 84 Wh example at 85% gives 71.4 Wh. At a 300 W load that is about 14 minutes.

If a battery’s data sheet lists capacity at more than one discharge rate, use the figure closest to how fast your load will drain it. The methodology page has the formulas both tools use. The load side of the same math is in how to find a device’s watts from its label.