The unit that counts
Milliamp-hours cannot tell you whether a battery is legal.
A power bank's headline number is almost always milliamp-hours, and the limit is written in watt-hours. The two are not interchangeable, because milliamp-hours describe charge and watt-hours describe energy, and converting between them needs the voltage the cells actually run at. A 25,000 mAh pack and a 26,250 mAh pack can land on opposite sides of a threshold depending on their cell chemistry. This is why a bigger mAh number is not automatically the less airline-friendly one.
Doing the arithmetic
Watt-hours are milliamp-hours times nominal voltage, divided by 1,000.
If a manufacturer publishes the nominal cell voltage, the conversion is arithmetic rather than estimation. Take the capacity in milliamp-hours, divide by 1,000 to get amp-hours, and multiply by the nominal voltage. The critical word is nominal: it is the average voltage across a discharge, usually 3.6 or 3.7 volts for a single lithium-ion cell, and it is not the 5 volts a USB port delivers. Using 5 volts inflates the answer by roughly a third and is the most common way people talk themselves into believing a battery is over the limit when it is not, or under it when it is not.
Reading a listing in four steps
- 01
Look for a watt-hour figure first
If the manufacturer prints a Wh rating, use it. It is the number the airline rule is written in and it needs no interpretation from you.
- 02
If there is none, look for the nominal voltage
A specification sheet that gives both milliamp-hours and a nominal voltage gives you everything the conversion needs.
- 03
Do not assume a voltage
Cell configuration changes the answer. A pack built from cells in series has a higher nominal voltage than a single cell, and guessing which one you are holding turns a legality question into a coin toss.
- 04
Check the printing on the battery itself
Many packs carry the rating on the case even when the product page omits it. If the number exists anywhere, it exists there.
What the catalog shows
Three of the four power banks here publish a rated energy. One does not.
The UGREEN Nexode 25K and the Anker Laptop 25K both publish 90 Wh. The Anker Prime 26K publishes 99.75 Wh, which clears the 100 Wh threshold by a quarter of a watt-hour and is the tightest margin in this catalog. The Anker Prime 20K publishes no watt-hour figure at all, and Anker does not publish a nominal voltage for it either, so the conversion above cannot be applied to it. Each figure and its absence is recorded on the product page with the source it came from.
Before you pack
The limit is per battery, not per bag.
Two 90 Wh packs are two separate batteries, each under the threshold, rather than one 180 Wh battery. Terminals should be protected against short circuits, which for a finished consumer power bank normally means nothing more than keeping it in a case or a pocket where the ports are not against metal. Rules on how many spares you may carry vary by airline, and the safest assumption is that a member of ground staff will ask about a large pack rather than that nobody will notice it.
