Battery Life Calculator: mAh, Wh, Hours
This battery life calculator estimates how long a battery will run a device. Enter the capacity in mAh and the current draw in mA, or the capacity in watt-hours and the power in watts for things like power banks, laptops, inverters and solar batteries. A derating factor accounts for real-world losses, and a duty-cycle option averages a device that sleeps most of the time and wakes up briefly, which is common for sensors and Arduino projects. Results show hours, days and the average current, so you can choose the right battery size.
Battery Life Calculator
Estimate how long a battery lasts from its capacity and the load current or power.
How to use the Battery Life Calculator
- Choose mAh and mA, watt-hours and watts, or duty cycle.
- Enter the battery capacity and the load.
- Set the efficiency or derating, usually 70% to 90%.
- Read the runtime in hours and days.
Battery life formula
Hours = capacity (mAh) × efficiency ÷ average current (mA), or capacity (Wh) × efficiency ÷ load (W). Wh = mAh × V ÷ 1,000.
Features
- mAh / mA and Wh / W modes
- Sleep and active duty cycle
- Efficiency derating
- Hours, days and average current
- Converts mAh to Wh at any voltage
- Runs in your browser
Frequently asked questions
How do I calculate battery life?
Battery life in hours = capacity (mAh) ÷ load current (mA) × efficiency. A 2,000 mAh battery powering a 100 mA load at 85% lasts about 17 hours.
How do I convert mAh to watt-hours?
Wh = mAh × volts ÷ 1,000. A 10,000 mAh power bank at 3.7 V holds 37 Wh.
Why use an efficiency factor?
Batteries rarely deliver their full rated capacity. Voltage converters, temperature, age and high current all reduce usable capacity.
How do I estimate a device that sleeps?
Average current = (active mA × active time + sleep mA × sleep time) ÷ total time. Use the duty cycle mode for this.
How long will a 100Ah 12V battery run a 100 W load?
100 Ah × 12 V = 1,200 Wh. With 85% efficiency, it runs a 100 W load for about 10 hours. Lead-acid should not be fully drained, so plan on about half.