Power banks explained: capacity, watts and flying with one

A power bank is one of the most useful things you can carry, but the numbers on the box are easy to misread. A bigger mAh figure does not always mean a better choice, and the number that decides how fast your phone or laptop charges is printed somewhere else. This guide explains the three numbers that matter — capacity, watt-hours and watts — and the rules for taking a power bank on a plane.

Capacity: what mAh really tells you

Power banks are sold by capacity in milliamp-hours (mAh), for example 10,000 or 25,000 mAh. That figure describes the battery cells inside, at their own internal voltage. Your phone or laptop charges at a different voltage, and some energy is always lost as heat while the power bank converts between the two. That is why you never get the full rated capacity into your device.

In practice, a 10,000 mAh power bank charges a typical modern phone about twice, not three or four times. Use mAh to compare power banks with each other, and use the manufacturer's "number of charges" for a specific phone as a rough real-world guide.

Watt-hours: the number airlines care about

Watt-hours (Wh) measure the energy stored in the battery. Many newer batteries print the Wh rating on the label; if yours does not, you can calculate it. The US Federal Aviation Administration explains that you multiply the battery voltage by its capacity in amp-hours. Lithium-ion cells are usually rated at about 3.6–3.7 V, so:

• a 10,000 mAh (10 Ah) power bank holds roughly 36–37 Wh;
• a 25,000 mAh power bank holds roughly 90–93 Wh.

Both are under the 100 Wh limit that matters for flying.

Output: what decides charging speed

How quickly a power bank charges your device depends on its output in watts (W), not its capacity. Most phones charge quickly from about 20–30 W. Laptops that charge over USB-C usually need 45–65 W, and larger laptops 100 W or more.

Look for USB Power Delivery (USB PD), the USB standard that lets a charger and a device agree on the right voltage and current. According to the USB Implementers Forum, USB PD was originally limited to 100 W using USB-C cables rated at 5 A, and the newer revision can deliver up to 240 W over full-featured USB-C cables. For a power bank, that means two things: check the output of the specific port you will use, and use a cable rated for the power you need.

Ports, cables and recharging

USB-C ports that work in both directions are the most convenient: the same port charges your devices and recharges the power bank. Built-in cables are handy because you cannot forget them. Also check the input rating — a power bank that accepts 30 W or more refills much faster than one limited to 10 W.

Flying with a power bank

The FAA's rules for passengers are clear:

• Spare lithium batteries, including power banks, must be in carry-on baggage only. If your carry-on is checked at the gate, take the power bank out and keep it with you.
• Lithium-ion batteries are limited to 100 Wh each. With airline approval you may carry up to two larger spares between 101 and 160 Wh.
• Protect the terminals from short circuits, for example in a pouch or the original packaging.
• Many airlines set stricter limits, including how many power banks each passenger may bring, so check with your airline before you travel.

Safety

All lithium-ion batteries can overheat. The FAA advises passengers to tell the crew immediately if a battery or device is overheating, swelling, smoking or burning, and damaged or recalled batteries must not be carried at all. Buy from established brands, avoid power banks that are swollen or damaged, and do not leave one in a hot car.

A quick checklist

1. Choose capacity for your day: about 10,000 mAh for a phone, 20,000 mAh or more for a laptop or a long trip.
2. Match the output in watts to your devices, and prefer USB PD over USB-C.
3. Check the Wh rating if you fly, and keep the power bank in your carry-on.
4. Look at the input rating too, so recharging does not take all night.

Picks to start your search

These power banks and chargers fit the checklist above; everyday tech essentials has more.