mAh to Wh converter
Convert the capacity of a battery or power bank from milliamp-hours (mAh) to watt-hours (Wh), or the other way round. You also see whether it is allowed on a plane.
In mAh or in Wh, depending on your choice.
Empty = 3.7 V, the cell voltage of lithium-ion batteries.
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How it's worked out
How it works
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How to convert mAh to Wh
Wh = mAh × V ÷ 1000. A 20,000 mAh power bank at 3.7 V holds 20,000 × 3.7 ÷ 1000 = 74 Wh. The other way round: mAh = Wh × 1000 ÷ V.
mAh only measures charge; you need the voltage to know how much energy is stored. That is why batteries with different voltages can only be compared in Wh.
Which voltage?
Lithium-ion cells have a nominal voltage of 3.6 to 3.7 V (3.85 V in many modern phones). Power banks quote their mAh at that cell voltage, not at the 5 V of the USB output. If you leave the voltage empty, we use 3.7 V.
Can I take the battery on a plane?
For lithium batteries in hand luggage, the limits (ICAO/IATA) are:
- up to 100 Wh: allowed;
- 100 to 160 Wh: only with the airline's approval;
- above 160 Wh: not allowed.
A 27,000 mAh power bank (99.9 Wh) just stays under the limit; a 30,000 mAh one (111 Wh) needs approval. Power banks always go in your hand luggage, never in the hold. Rules can differ per airline, so check before you fly.
Capacities to compare
| Device | mAh | Voltage | Wh |
|---|---|---|---|
| Phone battery | 5,000 | 3.85 V | 19.3 |
| Power bank | 10,000 | 3.7 V | 37 |
| NiMH AA cell | 2,000 | 1.2 V | 2.4 |
| E-bike pack | 10,000 | 36 V | 360 |
A laptop battery of 60 Wh is about 16,200 mAh at 3.7 V, which is why laptops are rated in Wh.
Phone charges from a power bank
A phone battery of 4,500 mAh at 3.85 V holds 17.3 Wh. A 74 Wh power bank at about 70% efficiency delivers 52 Wh, which is three phone charges, not four. The calculation is simple: usable Wh divided by the phone's Wh.
Reading the battery label
If only mAh is on the label, assume 3.6 or 3.7 V for lithium-ion. If the label shows Wh, use that. Travelling with several batteries: the airline limits apply to each battery separately, so check every one. For the cost to charge one, see the electricity cost calculator, and for the power of a charger the amps to watts calculator.
Frequently asked questions
Why does a 20,000 mAh power bank not charge my phone four times?
The mAh figure is at the cell voltage of 3.7 V, but the output is 5 V, and conversion loses 20 to 35% of the energy. A 74 Wh bank gives about 50 to 60 Wh, and a phone battery of 17 Wh then gets about three full charges.
What is Wh and how does it relate to kWh?
Watt-hours measure energy: 1 Wh is one watt for one hour. 1000 Wh is 1 kWh, the unit on your electricity bill.
How do I compare two batteries of different voltage?
Convert both to Wh. A 2000 mAh cell at 1.2 V stores 2.4 Wh, a 3000 mAh cell at 3.7 V stores 11.1 Wh, so the second holds far more even though the mAh figures are close.
How long will it take to charge a bank?
Divide the capacity in Wh by the charger's power in watts, and add 15 to 25% for losses. 74 Wh with an 18 W charger takes about 4.1 hours plus losses, roughly 5 hours.