Mobile & Electronics

Wh to Ah Calculator

Watt-hours into amp-hours at a chosen system voltage - for turning an energy requirement into a battery you can buy.

4 inputs Free, no sign-up

Your figures

The watt-hours the system has to store.

The voltage the battery bank runs at. Doubling it halves the amp-hours needed for the same energy.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

Energy requirements come in watt-hours; batteries are sold in amp-hours. This converts between them at whatever voltage your system runs at.

The system voltage is a design decision with real consequences. 1,200 Wh needs a 100 Ah battery at 12 V or a 50 Ah one at 24 V — same energy, half the amp-hours, and half the current in the cabling for a given load.

How to use this calculator

  1. Enter the energy the system has to store, in watt-hours or kilowatt-hours.
  2. Enter the system voltage — 12 V, 24 V or 48 V for most inverter installations.
  3. Read the amp-hours. That is the battery rating to shop for, before any allowance for depth of discharge.

The formula

Ah  = Wh / V
mAh = Wh x 1000 / V

Higher system voltages need fewer amp-hours for the same energy, and carry less current for the same load, which means thinner cable and smaller losses. That is why larger installations run at 24 V or 48 V rather than 12 V.

SymbolMeaningUnit
Wh Energy to store Wh
V System voltage V
Ah Nominal capacity needed Ah

Worked example

1,200 Wh of storage at 12 V:

Ah = 1200 / 12 = 100 Ah

The same 1,200 Wh at 24 V:

Ah = 1200 / 24 = 50 Ah

Notes and limits

  • This is the nominal capacity to store that energy. For lead-acid, buy roughly twice it, because discharging below about half repeatedly ruins the battery.
  • Lithium batteries tolerate deeper discharge, so the gap between nominal and usable is much smaller — which is part of why they cost more per Ah and less per usable Ah.
  • A higher system voltage means less current for the same power, so thinner cable and lower losses. It also means more batteries in series.
  • Batteries in a series bank should be matched in age and capacity, since the weakest one limits the whole string.
  • To size from a load and a runtime rather than from an energy figure, use the required capacity calculator.

What this calculation assumes

  • The result is nominal capacity, with no allowance for depth of discharge.
  • The system runs at the single voltage entered.
  • Lead-acid needs roughly twice this; lithium is closer to it.

Frequently asked questions

How do I convert Wh to Ah?

Divide the watt-hours by the system voltage. 1,200 Wh at 12 V is 100 Ah.

Should I use 12 V, 24 V or 48 V?

Higher voltages need fewer amp-hours and carry less current for the same power, so cabling is thinner and losses lower. 12 V suits small systems; 24 V and 48 V are usual once loads get into the kilowatts.

Do I buy the amp-hours this gives me?

For lithium, roughly yes. For lead-acid, buy about twice, because discharging one below half its capacity repeatedly shortens its life sharply.

Can I mix batteries of different capacities?

Not in series - the weakest battery limits the string and gets damaged first. Batteries in a bank should be matched in type, capacity and age.

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