About the Battery Life Calculator
This battery life calculator estimates how long a battery will run a device. Enter the battery capacity in milliamp-hours, amp-hours or watt-hours, its nominal voltage, and the load as a current (mA or A) or power (W), and it returns the expected runtime in hours and days along with the usable energy and the load in both amps and watts.
It suits electronics projects on Li-ion cells or AA packs, choosing a power bank for a phone or camera, sizing a 12 V deep-cycle battery for a camper, fridge or trolling motor, and estimating how long a portable power station will run a CPAP or laptop. A chart shows how runtime changes if the load is lighter or heavier.
Real batteries rarely deliver their full label capacity: lead-acid batteries should only be discharged to about 50% for long life, lithium packs lose some energy in voltage converters, and capacity falls in the cold and at high discharge rates. The "usable capacity" setting covers all of these — 80–90% is typical for lithium, 50% for lead-acid.
With the default inputs, the estimated runtime is 12 hours. Change any value above to recalculate instantly.
How to use the battery life calculator
- 1Enter the battery capacity from its label and choose mAh, Ah or Wh.
- 2Enter the nominal battery voltage.
- 3Enter the device’s average draw in mA, A or W.
- 4Set the usable capacity (80–90% lithium, about 50% lead-acid).
- 5Read the runtime and check the chart for other load levels.
Formula and method
Battery capacity is the charge (amp-hours) or energy (watt-hours) the battery stores. Dividing the usable part of that capacity by the rate the device consumes it gives the runtime. When both capacity and load are given as charge and current (mAh and mA, or Ah and A) the voltage cancels out; when either is given in watt-hours or watts, the calculator converts using Wh = Ah × V and W = A × V.
The usable-capacity percentage accounts for depth-of-discharge limits, DC-DC or inverter losses, battery ageing and temperature. This is a steady-load estimate; devices with bursts of high current (radios, motors) should use their average current, and very high discharge rates reduce effective capacity further (Peukert effect in lead-acid).
- Ah
- Capacity in amp-hours (mAh ÷ 1000)
- Wh
- Energy in watt-hours (Ah × V)
- A
- Average load current
- W
- Average load power
- usable%
- Fraction of rated capacity you can actually use
Worked examples
3,000 mAh Li-ion cell powering a 200 mA device
3,000 mAh × 80% = 2,400 mAh usable. 2,400 ÷ 200 = 12 hours. At 3.7 V the usable energy is 2.4 Ah × 3.7 = 8.88 Wh and the load is 0.2 × 3.7 = 0.74 W.
100 Ah 12 V deep-cycle battery running a 60 W fridge
100 Ah × 12 V = 1,200 Wh; using 50% for lead-acid leaves 600 Wh. 600 ÷ 60 W = 10 hours at a 5 A draw.
20,000 mAh power bank charging a 10 W device
Power banks are rated at the 3.7 V cell voltage: 20 Ah × 3.7 V = 74 Wh. With 85% surviving the 5 V conversion, 62.9 Wh ÷ 10 W ≈ 6.3 hours.
1,000 Wh power station running a 150 W load
1,000 Wh × 90% inverter efficiency = 900 Wh available. 900 ÷ 150 W = 6 hours.
Frequently asked questions
How long will a 100Ah battery last?+
Divide the usable amp-hours by the load current. A 100 Ah lead-acid battery used to 50% gives 50 Ah, so a 5 A load runs about 10 hours; a 100 Ah lithium battery at 90% would run the same load about 18 hours.
How do I convert mAh to Wh?+
Multiply mAh by the voltage and divide by 1,000. A 5,000 mAh battery at 3.7 V holds 5,000 × 3.7 ÷ 1,000 = 18.5 Wh.
Why does my battery not last as long as calculated?+
Capacity drops with age, cold temperatures and high discharge rates, and converters waste 10–20% of the energy. Devices also often draw more than their average rating during peaks. Lower the usable percentage to allow for this.
What does usable capacity mean?+
It is the share of the rated capacity you can realistically draw. Lead-acid batteries last longest if discharged only to 50%; lithium iron phosphate can use 80–100%. Converter losses also reduce what reaches the device.
How many hours is 3000mAh?+
It depends on the load: 3,000 mAh lasts about 30 hours at 100 mA, 15 hours at 200 mA and 3 hours at 1 A, before any allowance for losses.