How long will your battery really last? Enter the capacity and the power your device uses, and this free calculator estimates the runtime. It works for power banks, portable power stations, UPS batteries, car batteries and laptops.
Battery Runtime Calculator
Find out how long a battery will power your device. Pick a preset or enter your own numbers.
Battery runtime FAQ
How do I calculate battery runtime?
Convert the battery to watt-hours (mAh divided by 1,000, times volts), multiply by the usable percentage and efficiency, then divide by the device power in watts. The result is the runtime in hours.
Why is my power bank runtime shorter than the label suggests?
Power bank capacity is rated at the internal battery voltage, usually 3.7 V. Delivering power at 5 V or higher involves conversion losses, so you typically get roughly 60 to 75 percent of the rated energy.
What is the difference between mAh, Ah and Wh?
Milliamp-hours (mAh) and amp-hours (Ah) measure charge, while watt-hours (Wh) measure energy. Wh is the fairest way to compare batteries because it already includes voltage. Multiply Ah by volts to get Wh.
How much of a lead-acid battery can I use?
Most lead-acid batteries last longer if you discharge them only about halfway. Lithium iron phosphate and other lithium batteries can usually be used much deeper, often 80 to 100 percent.
The battery runtime formula
Runtime (hours) = battery energy (Wh) × usable % × efficiency % ÷ device power (W). If your battery is rated in mAh or Ah, convert it to watt-hours first: mAh ÷ 1,000 × volts, or Ah × volts.
Why real runtime is shorter than the label
- Conversion losses: power banks and inverters waste some energy as heat.
- Usable capacity: lead-acid batteries should not be drained fully, so only about half is usable.
- Temperature and age: cold weather and old cells reduce capacity.
- Peak vs average load: devices rarely draw their maximum rated power all the time.
How to find your device power
Check the label on the device or its power adapter for watts. If you only see volts and amps, multiply them. A plug-in power meter shows the real draw, which is often lower than the label.
Tips to get more runtime
- Power DC devices directly instead of through an inverter when you can.
- Turn off screens, lights and idle devices you do not need.
- Choose a larger battery in watt-hours rather than relying on mAh figures alone.
- Keep batteries out of extreme heat and cold.