Charge versus energy
A milliamp hour is a quantity of charge: a 1,000 mAh battery can supply 1,000 milliamps for one hour, or 100 milliamps for ten. It says nothing about voltage. A watt hour is energy: charge multiplied by the voltage it is delivered at. Since every device draws energy, not charge, Wh is the figure that compares batteries of different kinds; mAh only compares batteries of the same voltage. That is why laptops, tools and airline rules use Wh, and phones and power banks, whose cells all sit at roughly the same voltage, use mAh.
Wh = mAh × V ÷ 1,000. mAh = Wh × 1,000 ÷ V.
The voltage to use is the battery's nominal voltage, 3.7 V for the lithium ion cells in phones and power banks unless the label says otherwise. A power bank that is also labelled in Wh has already done this sum, and that printed figure is the one that counts. The energy converter relates watt hours to joules and kilowatt hours, and the volts, amps and watts guide explains the quantities underneath.
Common power banks in watt hours
| Label | Energy at 3.7 V | On a plane |
|---|---|---|
| 5,000 mAh | 18.5 Wh | Carry-on, no approval needed |
| 10,000 mAh | 37.0 Wh | Carry-on, no approval needed |
| 20,000 mAh | 74.0 Wh | Carry-on, no approval needed |
| 26,800 mAh | 99.2 Wh | Carry-on, no approval needed |
| 27,000 mAh | 99.9 Wh | Carry-on, no approval needed |
| 30,000 mAh | 111.0 Wh | Carry-on with airline approval, max two |
| 50,000 mAh | 185.0 Wh | Not allowed |
The 100 Wh line falls at 27,027 mAh and the 160 Wh line at 43,243 mAh, which is why so many large power banks are sold at 26,800 or 27,000 mAh: they sit just under the first limit.
The rule for planes
US hazardous materials regulation 49 CFR 175.10 sets the limits that airlines and TSA apply:
- Up to 100 Wh: allowed in equipment and as spares, no approval needed.
- Over 100 Wh up to 160 Wh: allowed with the operator's approval; no more than two such spare batteries per person, in carry-on baggage.
- Over 160 Wh: not allowed as passenger baggage.
- Spare batteries, which includes power banks: carry-on baggage only, each individually protected against short circuit, for instance in its packaging or with the terminals taped.
- Devices in checked bags: must be completely switched off, not asleep or hibernating; very small batteries are exempt.
- Lithium metal batteries (the non-rechargeable kind): lithium content up to 2 grams.
TSA's summary adds the practical point: spare batteries over 100 Wh are a carry-on item with airline approval and are never allowed in checked bags. Airlines outside the US apply the same 100 and 160 Wh thresholds under international rules, but check the carrier's own page, since approval procedures differ.
Why the mAh on the box never arrives in full
A 10,000 mAh bank and a phone with a 4,400 mAh battery look like 2.3 charges. In energy they are 37 Wh and 16.3 Wh, the same ratio, but the bank's 3.7 V cells have to be stepped up to the 5 V or more of the charging cable and then stepped back down inside the phone, and each conversion wastes some energy as heat. The real count is lower than the ratio, and how much lower depends on the bank's electronics and the cable. The honest comparison is Wh to Wh with a margin, never mAh to mAh.
What a charge costs
Energy is cheap at this scale. Filling a 37 Wh bank from the wall draws about 0.037 kWh plus charger losses, which the electricity cost calculator prices at a fraction of a cent at any tariff; the power converter turns a charger's wattage into the time a full charge needs, and the unit converter covers the rest of the electrical units.
Frequently asked questions
What is the difference between mAh and Wh?
Milliamp hours count charge: how many milliamps a battery can deliver for an hour. Watt hours count energy: charge times voltage. Two batteries with the same mAh but different voltages hold different amounts of energy, which is why airlines, laptops and electric tools are rated in Wh while phones and power banks advertise mAh at a cell voltage of 3.7 volts.
How do I convert mAh to Wh?
Multiply the mAh by the battery voltage and divide by 1,000. A 10,000 mAh power bank at 3.7 V holds 37 Wh. If the label gives a different nominal voltage, use that; if it gives Wh directly, that figure is the one the airline rule uses.
Can I take a power bank on a plane?
Only in carry-on baggage, and under a size limit. US regulation 49 CFR 175.10 allows lithium ion batteries up to 100 Wh without approval, batteries of more than 100 Wh and up to 160 Wh with the airline's approval and at most two spares per person, and nothing larger. Spare batteries, which a power bank is, must be in carry-on baggage only and protected against short circuits. At 3.7 V, 100 Wh is about 27,027 mAh.
Why does a 10,000 mAh bank not charge my phone twice?
Because the bank's cells are at 3.7 V and the phone is charged at 5 V or more through a converter that loses some energy as heat, and the phone's own charging circuit loses more. Compare in Wh and allow for the losses: a 37 Wh bank into a 16.3 Wh phone battery is 2.3 charges before losses, and fewer after.
Do devices in checked bags have to be off?
Yes. The regulation says devices with lithium batteries in checked baggage must be completely switched off, not in sleep or hibernation, with an exception for very small batteries. Spare batteries and power banks may not go in checked baggage at all.
Sources
- 49 CFR 175.10(a)(18): lithium batteries carried by passengers and crew (lithium ion batteries up to 100 Wh; with operator approval over 100 Wh up to 160 Wh and no more than two such spares per person in carry-on; lithium metal batteries up to 2 g lithium content; spare batteries in carry-on only and individually protected against short circuit; devices in checked baggage completely switched off)
- Transportation Security Administration: Lithium batteries with more than 100 watt hours (carry-on with airline approval, up to two spare batteries of 101 to 160 Wh, not in checked bags)