How to use the battery life and mah to wh calculator
- Enter the battery capacity in mAh, Ah or Wh and the nominal voltage from the label.
- Enter the load the device draws and its unit: milliamps or amps at the battery voltage, or watts.
- Adjust the usable share of the capacity and the converter efficiency. The defaults are example values.
- Use the advanced fields for packs wired in series or parallel, then read the estimated run time and the energy in watt-hours.
Formula
Energy (Wh) = mAh x V / 1000 = Ah x V
Usable energy = energy x usable share
Load power (W) = amps x volts
Run time (h) = usable energy / (load power / efficiency)
Series packs multiply the voltage, parallel packs multiply the capacity, and in both cases the energy is multiplied by the number of batteries.
Worked example
A 5,000 mAh battery at 3.7 V stores 5,000 x 3.7 / 1,000 = 18.5 Wh. With 80% usable the energy is 14.8 Wh. A device that draws 250 mA from it uses 0.925 W, and a converter at 90% efficiency means the battery supplies 1.028 W.
The run time is 14.8 / 1.028 = 14.4 hours, or 14 h 24 min. A 100 Wh pack feeding a 50 W load with no losses runs exactly 2 hours, and four such packs in a series and parallel array run 8 hours.
mAh versus Wh
Milliamp hours measure charge, not energy, so a 5,000 mAh battery at 3.7 V holds less energy than a 5,000 mAh battery at 12 V. Watt-hours multiply capacity by voltage, which makes batteries of different voltages comparable. Airlines also use Wh to limit lithium batteries, so it is worth knowing the figure for your own pack.
Why real run time is shorter
Batteries rarely deliver their full rating. High loads, cold weather, age and the cutoff voltage of the device all reduce usable capacity. Many devices also draw different power over time, so use an average. The usable share and efficiency here are example values to adjust, based on your own measurements if you have them.
Related tools
To work out the current in a circuit see the Ohm's law calculator, and to size a resistor for an LED run from a battery use the LED resistor calculator. Electricity costs for charging are in the electricity cost calculator.
Assumptions and limits
- The voltage is a nominal figure and is assumed constant, which is not true as a battery discharges.
- The load is a steady average. Devices with sleeping and active modes need an average that you work out.
- Usable share and converter efficiency are example values.
- Self discharge, temperature effects and battery aging are not modeled.
Frequently asked questions
How do I convert mAh to Wh?
Multiply the mAh by the battery voltage and divide by 1,000. A 5,000 mAh battery at 3.7 V is 18.5 Wh.
How do I estimate battery life?
Divide the usable energy in watt-hours by the load power in watts. Allow for losses in any converter between the battery and the device.
Why do two batteries with the same mAh store different energy?
Because their voltages differ. Energy depends on both capacity and voltage, so compare watt-hours.
What happens when I connect batteries in series or parallel?
Series adds the voltages and keeps the capacity. Parallel adds the capacity and keeps the voltage. Either way the stored energy adds up.