Battery Life Calculator
Enter the battery capacity and how much power the device draws. You get the running time in hours and days. Or switch to find the battery size for the time you want.
This is an estimate. Cold weather, an ageing battery and heavy loads all shorten the time.
Worked examples
A small battery
5,000 mAh at 3.7 V, 500 mA load, 85% usable
8.5 hours
The battery holds 18.5 Wh and the load is 1.85 W.
About Battery Life Calculator
A battery life calculator works out how long a battery will run a device. The energy in a battery in watt hours is its capacity in amp hours times its voltage, so a 5,000 mAh battery at 3.7 volts holds 18.5 watt hours. The running time is that energy divided by the power the device draws, and a device taking 500 milliamps at 3.7 volts uses 1.85 watts.
No battery gives up all its energy. Some is lost in the circuit that raises or lowers the voltage, and a battery should not be run completely flat, so the tool uses a usable share, 85% by default. At that setting, the example above runs for 8.5 hours. Switch to the other mode to find the battery size for a run time you want, which is useful for a project with a camera, a sensor or a small pump.
How to use Battery Life Calculator
- Choose whether you want the run time or the battery size.
- Enter the capacity in mAh and the voltage.
- Enter the power the device draws, in milliamps or watts.
- Adjust the usable share and read the result.
Common uses
- Estimating a power bank or camera runtime
- Sizing a battery for a project
- Comparing batteries
- Checking a smart device's claim
Frequently asked questions
How do I work out battery life?
Convert the capacity to watt hours by multiplying amp hours by volts, then divide by the power in watts. 18.5 watt hours at 1.85 watts is 10 hours before losses, or 8.5 hours at 85%.
What do mAh and Wh mean?
mAh is milliamp hours, a measure of charge, and Wh is watt hours, a measure of energy. To turn mAh into Wh, multiply by the voltage and divide by 1,000.
Why is the real time shorter?
Cold weather, ageing, heavy loads and voltage conversion all cut the energy you can use. Lowering the usable share gives a cautious figure.
How big a battery for 24 hours?
A device drawing 500 mA at 3.7 V and a usable share of 85% needs 52.2 Wh, or about 14,118 mAh, to run for 24 hours.
Can I use this for larger batteries?
Yes. Enter the capacity in mAh, for example 100,000 for a 100 Ah battery, the voltage of the battery, and the load in watts.