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Battery life calculator: mAh to Wh, power bank and charge time

Your battery
Typical voltage

Most lithium batteries state their mAh at 3.7 V, the nominal voltage of one cell. If no voltage is printed, keep 3.7 V.

85% is a rough figure for a power bank: some energy is lost as heat. The reserve stops you from running down to 0% (10 to 20% is kinder to the battery); enter 0 to use everything.

Typical device

The suggested values are rough figures: replace them with your device's own (label, manual).

How it is worked out

Safety: keep batteries away from heat (a car in the sun, a radiator) and knocks, use a suitable charger, and stop using a battery that is swollen, damaged or gets unusually hot.

Your entries are saved on this device.

How long will your battery last?

This battery life calculator starts from the capacity printed on a battery (in mAh, Ah or Wh) and its voltage. It gives you the energy in watt-hours, how long a device will run, how many times a power bank can charge your phone, a rough charge time and where the battery sits against the Wh limits used by airlines. "Copy link" reopens the calculation with your values.

How to use it

  1. Under "Your battery", enter the "Capacity", the "Unit" and the "Voltage (V)" ("3.7 V (lithium)" button).
  2. Adjust the "Efficiency (%)" and the "Reserve to keep (%)".
  3. Pick a tab: "Battery life" (consumption in W or mA, or a "Typical device"), "Number of charges", "Charge time" or "Flying".

mAh, Wh and voltage in plain terms

mAh (milliamp-hours) measure an amount of charge, not energy. To get the energy you need the voltage: Wh = mAh × V ÷ 1,000. A 20,000 mAh power bank at 3.7 V therefore holds 20,000 × 3.7 ÷ 1,000 = 74 Wh, or 20 Ah. Watt-hours let you compare batteries with different voltages, so the mAh to Wh conversion appears as soon as you type.

Why a 10,000 mAh power bank does not deliver 10,000 mAh

Power bank ratings are given at 3.7 V, the voltage of the lithium cells inside. The USB output, though, runs at 5 V. A 10,000 mAh bank holds 37 Wh, which is only 37 ÷ 5 × 1,000 = 7,400 mAh at 5 V. On top of that, some energy turns into heat. With 85% efficiency, a common rough figure, 31.45 Wh are left, about 6,290 mAh at 5 V.

A worked example: a phone with a 4,000 mAh battery at 3.85 V stores 15.4 Wh. The power bank charges it 31.45 ÷ 15.4 = 2.04 times, not the 2.5 times that 10,000 ÷ 4,000 suggests. Plugged into a 5 W device, it runs for 31.45 ÷ 5 = 6.29 hours, or 6 hr 17 min. If a gadget lists 500 mA at 5 V, the calculator treats it as 500 × 5 ÷ 1,000 = 2.5 W.

A tablet with 7,500 mAh at 3.8 V stores 28.5 Wh: the same bank charges it 31.45 ÷ 28.5 = 1.10 times. A 45 W cool box on a 600 Wh camping battery at 85% runs for 600 × 0.85 ÷ 45 = 11.33 hours.

Choosing a power bank

Work back from what you need to charge. Two full charges of a 15.4 Wh phone at 85% efficiency call for at least 15.4 × 2 ÷ 0.85 = 36.2 Wh, so a 10,000 mAh bank at 3.7 V will do. For a laptop, also check that the output can supply the wattage it needs.

Battery charge time

The formula is capacity in Wh ÷ (charger power × efficiency). A 5,000 mAh battery at 3.7 V (18.5 Wh) on an 18 W charger at 80% efficiency receives 14.4 W: about 18.5 ÷ 14.4 = 1.28 hours, or 1 hr 17 min. The end of the charge is slower. Past roughly 80%, the charger holds the voltage steady and the current tapers off (CC/CV charging), which often adds 20 minutes to an hour.

Flying with a power bank

Airlines commonly apply two limits: up to 100 Wh, a battery generally goes in your carry-on without any formalities; between 100 and 160 Wh, airline approval is often required; above that, it is generally refused. Power banks and spare batteries travel in the cabin, never in checked baggage as a rule. A 27,000 mAh bank at 3.7 V comes to 99.9 Wh, just under the first limit. These are guidelines only: check your airline's rules before you fly.

Looking after your batteries

Keep batteries out of the heat (a car in the sun) and, as a general rule, avoid leaving them at 0% or 100% for long. Use a suitable charger and retire a swollen or damaged battery.

Frequently asked questions

How do I convert mAh to Wh?

Multiply by the voltage, then divide by 1,000: 3,000 mAh at 3.7 V is 11.1 Wh.

Which voltage should I use if none is printed?

For a lithium battery, 3.7 V is the most common nominal voltage. Recent phones often state 3.85 V.

Why is real battery life shorter than the estimate?

A device's power draw changes with use, and batteries lose capacity as they age and in cold weather. Treat the result as an estimate.

What does the reserve do?

It takes out the share you do not want to use. With a 20% reserve, 37 Wh at 85% gives 25.16 Wh of usable energy.