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Battery Runtime Calculator

Enter the capacity of a battery and the power a device draws to estimate how many hours it will run. The capacity can be in Wh, mAh or Ah, and the load in watts, milliamps or amps. You can also reverse the question and find the capacity needed to run for a number of hours.

Estimated runtime

3 h 42 min

3.7 h · usable energy 18.5 Wh · draw on the battery 5 W

Adjust for real conditions

The rated capacity is rarely all available: it falls with cold, age and a heavy discharge, and this page does not model any of that. Enter the share you count on. If a converter sits between the battery and the load, for example a step-up to 5 V, part of the energy is lost in it: enter its efficiency, or leave 100 when the load is wired directly. Empty means 100.

Capacity needed for a target runtime

Battery capacity needed

50 Wh

13513.5 mAh (13.5135 Ah) at 3.7 V

Runtime = energy × usable share × efficiency ÷ load. mAh × volts ÷ 1,000 gives Wh; mA × volts ÷ 1,000 gives W. This is not a safety or compliance check and does not replace the manufacturer's datasheet. Nothing you type leaves your browser.

How it works

  1. Enter the capacity and pick Wh, mAh or Ah. For mAh and Ah, enter the battery voltage too.
  2. Enter the load and pick W, mA or A. For mA and A, enter the voltage the device runs at.
  3. Read the runtime in hours and minutes, and the usable energy.
  4. Adjust the usable share and the converter efficiency if you want, or enter the hours you need to see the capacity required.

What people use it for

  • A power bank and a device

    See how long a power bank can feed a small device from its mAh rating and the watts the device draws.

  • A hobby or DIY project

    Size the battery of a sensor, a camera or a lamp from the current it draws.

  • Camping and travel gear

    Find the capacity a battery needs to run a small light or a fan for a whole night.

Questions

How is the runtime calculated?
Energy in Wh = capacity in Wh, or mAh × volts ÷ 1,000, or Ah × volts. Load in W = watts, or mA × volts ÷ 1,000, or A × volts. Runtime in hours = energy × usable share × converter efficiency ÷ load. A 5,000 mAh battery at 3.7 V holds 18.5 Wh; with a 5 W load that is 18.5 ÷ 5 = 3.7 hours.
What are the usable share and the efficiency for?
The usable share is the part of the rated capacity you count on. The efficiency is the share of the energy that reaches the load when a converter sits between the battery and the device, for example a step-up to 5 V; the rest is lost as heat. With 80 % usable and 90 % efficiency, 18.5 Wh at 5 W gives 18.5 × 0.8 × 0.9 ÷ 5 = 2.664 hours.
Why can the real runtime be shorter?
The real capacity of a battery falls with cold, age and a heavy discharge, and its voltage changes as it empties. The page does not model any of this: it divides the energy you enter by the power you enter. Use a lower usable share to keep a margin.
Does it check that a battery is safe or suitable?
No. It is not a safety or compliance check and does not replace the manufacturer's datasheet. The limits of current, voltage and temperature come from the maker of your battery and of your device.