# What is the battery life?

Works out how long a battery runs a load: capacity in mAh ÷ current in mA, or capacity in Wh ÷ power in W, with an optional share of the capacity that is usable.

- Page: https://www.acalculator.org/physics/battery-life-calculator
- JSON spec: https://www.acalculator.org/physics/battery-life-calculator.json
- Version: 0a47a8446677

## Default answer

Example with the default inputs (I know mAh and current, Capacity (mAh) 2,000, Load current 100 mA, Usable capacity 100%): The battery lasts about 20 hours (20 hours).

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| by | I know | The capacity in mAh and the current, or the capacity in Wh and the power. |
| mah | Capacity (mAh) | The battery’s rated capacity in milliampere hours (1 Ah = 1,000 mAh). |
| i | Load current | The average current the device draws. |
| wh | Capacity | The battery’s rated energy in watt hours (or volts × amp hours). |
| p | Load power | The average power the device uses. |
| u | Usable capacity | The share of the rated capacity you expect to get out: 100% for the label value; less for age, cold, high loads or a converter. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| hours | Battery life (hours) | Capacity × usable share ÷ load, in hours. |
| clock | In days, hours and minutes | The battery life rounded half up to the whole minute. |
| days | Battery life (days) | The hours ÷ 24. |

## Method

Hours = capacity (mAh) ÷ 1,000 × usable share ÷ load (A), or capacity (Wh) × usable share ÷ load (W). Days = hours ÷ 24.

## Assumptions

- A steady average load. Real batteries give less at high currents and low temperatures (Peukert’s law), and less as they age; lower the usable share to allow for this.
- mAh mode assumes the load runs at the battery’s own voltage. For a device behind a voltage converter, use Wh and watts.
- Exact on the numbers as typed; rounded only to show.

## Worked examples

1. by = current, mah = 2,000, i = 0.1, u = 100% gives hours = 20, clock = 20 hours, days = 0.833333. Source: Wikipedia, "Ampere hour" (a battery of C ampere hours supplies a current of I amperes for C ÷ I hours), https://en.wikipedia.org/wiki/Ampere_hour (retrieved 2026-10-05).
2. by = current, mah = 3,000, i = 0.25, u = 80% gives hours = 9.6, clock = 9 hours, 36 minutes. Source: Wikipedia, "Ampere hour" (a battery of C ampere hours supplies a current of I amperes for C ÷ I hours), https://en.wikipedia.org/wiki/Ampere_hour (retrieved 2026-10-05).
3. by = power, wh = 180,000, p = 12, u = 100% gives hours = 4.166667, clock = 4 hours, 10 minutes. Source: Wikipedia, "Kilowatt-hour" (energy = power × time; 1 Wh is 1 W for 1 hour), https://en.wikipedia.org/wiki/Kilowatt-hour (retrieved 2026-10-05).
4. by = current, mah = 220, i = 0.000015, u = 100% gives hours = 14,666.666667, clock = 611 days, 2 hours, 40 minutes. Source: Wikipedia, "Ampere hour" (a battery of C ampere hours supplies a current of I amperes for C ÷ I hours), https://en.wikipedia.org/wiki/Ampere_hour (retrieved 2026-10-05).

## FAQ

### How do I calculate battery life?

Divide the capacity by the load. In milliampere hours: hours = mAh ÷ mA, so a 2,000 mAh battery at 100 mA lasts 20 hours. In watt hours: hours = Wh ÷ W, so a 50 Wh battery at 12 W lasts about 4 hours 10 minutes.

### Why does my battery run out sooner than the calculator says?

The rated capacity is measured at a gentle current and room temperature. High currents, cold, age and the battery’s cut-off voltage all reduce what you get out. Set the usable capacity below 100%, for example 80%, to plan with a margin.

### Should I use mAh or Wh?

Use mAh when the device runs straight from the battery at its voltage and you know its current. Use Wh when a converter changes the voltage (phones, laptops and power banks with USB outputs), because watt hours count energy whatever the voltage.

### How do I turn mAh into Wh?

Multiply by the battery’s nominal voltage and divide by 1,000. A 3,000 mAh cell at 3.7 V holds 3,000 × 3.7 ÷ 1,000 = 11.1 Wh.

### How do I find a device’s average current?

Measure it with a multimeter in series, or read the power from the label and divide by the voltage. If the device sleeps and wakes, average it over time: 10 mA for 1% of the time and 0.01 mA the rest averages about 0.11 mA.

### What is Peukert’s law?

A rule that a battery gives less total charge the faster it is drained, which matters most for lead-acid batteries. The calculator does not model it; a lower usable share stands in for it.

## Sources

- Wikipedia, "Ampere hour": a battery of C ampere hours supplies I amperes for C ÷ I hours. https://en.wikipedia.org/wiki/Ampere_hour (retrieved 2026-10-05)
- Wikipedia, "Kilowatt-hour": energy = power × time; 1 Wh is 1 watt for 1 hour (3,600 J). https://en.wikipedia.org/wiki/Kilowatt-hour (retrieved 2026-10-05)
- Wikipedia, "Peukert’s law": capacity falls at higher discharge rates. https://en.wikipedia.org/wiki/Peukert%27s_law (retrieved 2026-10-05)