mAh to Wh Converter for Batteries and Power Banks
mAh to Wh formula: Wh = mAh × voltage (V) / 1000. A 10,000 mAh power bank at 3.7 V stores 10,000 × 3.7 / 1000 = 37 Wh. The mAh number alone says little, because batteries run at different voltages – only watt-hours can be compared.
Source: IATA, IATA – Passenger lithium battery table (Wh limits for spare batteries and power banks). Updated: .
How it is calculated
Formula: mAh to Wh and back
Milliamp-hours (mAh) describe the electric charge a battery can deliver. The stored energy in watt-hours (Wh) needs the voltage as well, because 1 Ah × 1 V = 1 Wh:
- Wh = mAh × V / 1000
- mAh = Wh × 1000 / V
1 Wh equals 3,600 joules (NIST SP 811). For power banks and phone batteries use the nominal cell voltage, usually 3.6 or 3.7 V – not the 5 V at the USB port.
Table: common power banks in Wh
| Capacity | at 3.6 V | at 3.7 V | Carry-on |
|---|---|---|---|
| 5,000 mAh | 18 Wh | 18.5 Wh | yes |
| 10,000 mAh | 36 Wh | 37 Wh | yes |
| 20,000 mAh | 72 Wh | 74 Wh | yes |
| 26,800 mAh | 96.5 Wh | 99.2 Wh | yes |
| 30,000 mAh | 108 Wh | 111 Wh | airline approval |
| 50,000 mAh | 180 Wh | 185 Wh | no |
Why a 10,000 mAh power bank doesn’t charge a phone 2.5 times
A 4,000 mAh phone seems to fit 2.5 times into 10,000 mAh. But both figures only hold at cell voltage, and energy is lost in the voltage converter (3.7 V → 5 V → 3.85 V), in the cable and as heat. That’s why this tool works in watt-hours with an adjustable efficiency. Many manufacturers also print a “rated capacity at 5 V” that already includes these losses.
Power banks on a plane
According to the IATA lithium battery table, power banks count as spare batteries: they always go in carry-on baggage, never in checked bags. Up to 100 Wh they are allowed without approval; from 100 to 160 Wh the airline must approve (two at most); above that they are forbidden in passenger baggage. If the power bank shows no Wh rating, calculate it here from mAh and voltage. Some airlines also restrict using power banks on board.
Frequently asked questions
How do you convert mAh to Wh?
Multiply mAh by the voltage and divide by 1000. Example: 20,000 mAh × 3.7 V / 1000 = 74 Wh. The voltage is printed on the battery; power banks usually use 3.6 or 3.7 V.
How many Wh is a 20,000 mAh power bank?
74 Wh at 3.7 V cell voltage, 72 Wh at 3.6 V. That is below 100 Wh, so it may go in carry-on baggage.
How many mAh is 100 Wh?
At 3.7 V, 100 Wh = 100 × 1000 / 3.7 ≈ 27,027 mAh; at 3.6 V ≈ 27,778 mAh. That’s why 26,800 mAh power banks sit just under the 100 Wh flight limit.
Why not use the 5 V from the USB port?
The mAh rating refers to the cells inside, which run at 3.6–3.7 V. Using 5 V would overstate the energy by about a third. The 5 V figure is only useful for the usable capacity at the output.
How long will a power bank run a device?
Usable watt-hours divided by the device’s power. 37 Wh × 85 % ≈ 31 Wh runs a 10 W device for about 3 hours. The power rating is on the charger or in the device specs.
Sources and legal basis
- IATA: IATA – Passenger lithium battery table (Wh limits for spare batteries and power banks)
- NIST SP 811 App. B.9: NIST SP 811 – Guide for the Use of the SI, Appendix B.9: watt hour = 3.6 × 10³ J
As of:
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