How long will a battery last? The formula
Battery life is the energy in the battery divided by the power the device uses. Everything else is unit conversion:
- Energy (Wh) = mAh × volts ÷ 1000, or Ah × volts, or the Wh printed on the battery.
- Usable energy = Wh × usable share × efficiency.
- Runtime (hours) = usable Wh ÷ load in watts.
When the battery and the current are at the same voltage, the shortcut is mAh ÷ mA: a 2000 mAh AA battery feeding a 50 mA gadget lasts about 40 hours. Mixing voltages is the classic mistake: a "10,000 mAh" power bank is rated at its 3.7 V cells, not at the 5 V or 9 V it delivers, which is why watt-hours are the safer unit to compare.
Battery runtime examples
| Battery | Load | Calculation | Runtime |
|---|---|---|---|
| Phone, 4500 mAh at 3.85 V | 1.5 W light use | 17.3 Wh ÷ 1.5 W | about 11.5 h |
| Laptop, 60 Wh | 12 W browsing | 60 ÷ 12 | 5 h |
| UPS, 12 V 9 Ah (108 Wh) | 100 W computer | 108 × 80% × 85% ÷ 100 | about 44 min |
| AA, 2000 mAh | 50 mA | 2000 ÷ 50 | 40 h |
These are typical values, not measurements of your device. A phone's draw changes a lot with screen brightness, signal and apps; check your phone's battery settings for its real average.
Watt hours calculator: mAh, Ah and Wh
Watt-hours are the one number that lets you compare batteries of different voltages. Multiply milliamp-hours by the voltage and divide by 1000: a 5000 mAh phone battery at 3.85 V holds 19.25 Wh, and a 10,000 mAh power bank at 3.7 V holds 37 Wh. To go back, divide Wh by the voltage and multiply by 1000. The converter above does both ways; it's also handy for checking a power bank against the common 100 Wh airline limit.
Power banks: how many phone charges?
A power bank's 37 Wh doesn't all reach your phone. Converting the cells' 3.7 V up to 5 V or 9 V and then charging the phone's battery loses energy, so expect roughly two thirds to arrive. That 10,000 mAh bank therefore charges a 4500 mAh phone (17.3 Wh) about 1.4 times, not 2.2 times as the mAh numbers suggest. Set the efficiency to about 65% when you calculate charges.
UPS and car battery runtime
For a UPS or a 12 V battery running a fridge or lights, the inverter or converter typically loses 10 to 15%, so 85% efficiency is a sensible start. Lead-acid batteries also deliver less than their rating at high loads and wear out quickly when emptied completely, so use a usable share of 50% for regular use or 80% for an emergency estimate. This battery life calculator estimates runtime; it can't tell how many years a car battery will last, which depends on heat, short trips and age.
Why real battery life is shorter
- Age: lithium batteries typically lose capacity over hundreds of cycles; an older phone may hold well under its original rating.
- Cold: below about 10 °C (50 °F) a battery delivers noticeably less.
- High current: fast discharge, especially from lead-acid and alkaline cells, gives less total energy.
- Cut-off: devices shut down before the battery is truly empty to protect it.
How we tested this tool
We checked the calculator against the worked examples in the table by hand calculation: 4500 mAh at 3.85 V is 17.33 Wh and lasts 11.55 h at 1.5 W; 60 Wh at 12 W is 5 h; 108 Wh with 80% usable and 85% efficiency at 100 W is 44 minutes; 2000 mAh at 50 mA is 40 h. We checked that mA and A loads are converted at the battery voltage, that the result is the same whether a battery is entered as 9 Ah at 12 V or as 108 Wh, that the mAh ⇄ Wh converter works in both directions (5000 mAh at 3.85 V ⇄ 19.25 Wh), and that empty or zero values show no result instead of a wrong one. We tested in a desktop browser and at a 375 px phone width. The presets are typical values, not measurements of any specific device.
Frequently asked questions
How do I calculate how long a battery will last?
Divide the usable energy by the power the device draws: hours = watt-hours × usable share × efficiency ÷ watts. If the capacity and the current use the same voltage, you can also divide mAh by mA: a 2000 mAh battery at 50 mA lasts about 40 hours.
How do I convert mAh to Wh?
Multiply milliamp-hours by the battery voltage and divide by 1000: Wh = mAh × V ÷ 1000. A 5000 mAh phone battery at 3.85 V holds 19.25 Wh.
Why does my battery run out sooner than calculated?
Rated capacity is measured under ideal conditions. Age, cold, high currents, voltage converters and the device shutting down before the battery is completely empty all reduce what you actually get.
What efficiency should I use?
Use 100% when the device runs straight from the battery. Use about 85% for a UPS or inverter and about 80 to 90% for a power bank powering a device at 5 V or USB-C. Charging another battery loses more, so a power bank often delivers only about two thirds of its rated watt-hours to a phone.
How much of a lead-acid battery can I use?
Lead-acid batteries last longer if you use only about half of their capacity, and their capacity shrinks at high loads. Set the usable share to 50% for long life or 80% for an occasional emergency run. Lithium batteries can usually use 90 to 100%.
Is this calculator accurate for car batteries?
It estimates how long a car battery can power an accessory, like a fridge or lights, from its Ah rating. It does not predict how many years a car battery will last; that depends on climate, driving and age.