Short answer: Carry it in hand luggage only. Anything up to 100 Wh (about 27,000 mAh) is allowed without asking, and 10,000 mAh is the right size for most people.
Most people buy a power bank by the biggest number on the box. That number, in mAh, matters less than you think, and the one that matters at the airport is printed much smaller.
Capacity: mAh versus watt-hours
The mAh figure describes the battery cells inside, usually at 3.7 volts. Airlines and real-world charging care about energy, measured in watt-hours (Wh).
To convert: Wh = mAh × 3.7 ÷ 1000.
- 10,000 mAh is about 37 Wh
- 20,000 mAh is about 74 Wh
- 27,000 mAh is about 99.9 Wh, the largest size most brands make for travel
You get less than the label says
Converting from the battery's voltage to your phone's loses energy as heat. Expect roughly two thirds of the rated capacity to reach your device. A 10,000 mAh bank will charge a phone with a 4,500 mAh battery about one and a half times, not twice.
Tip: For one phone over a long day or a short trip, 10,000 mAh is the sweet spot: light, cheap and enough for one to two full charges.
Charging speed
Look for USB-C Power Delivery (PD). A 20 W output fast-charges most phones. If you want to top up a laptop, you need 45 W or more, and the laptop must support USB-C charging.
Airline rules
These are the limits most airlines follow, based on international dangerous-goods rules:
- Power banks go in hand luggage only. Never put them in checked bags.
- Up to 100 Wh: allowed without asking.
- 100 to 160 Wh: usually needs airline approval, and you may carry at most two.
- Over 160 Wh: not allowed on passenger planes.
If the watt-hour rating is not printed on the power bank itself, security staff can refuse it, even if it is small.
Warning: A growing number of airlines now ban using or charging power banks during the flight, and some ask you to keep them out of overhead bins. Check your airline's rules before you fly.
How to pick one in a minute
- Choose the capacity. 10,000 mAh for a phone, 20,000 mAh for a phone plus earbuds or a tablet, up to 27,000 mAh if you also charge a laptop.
- Check the output. USB-C PD, at least 20 W for phones and 45 W or more for laptops.
- Find the Wh rating. Make sure it is printed on the case and under 100 Wh if you travel by air.
- Buy from a known brand. Cheap unbranded banks often overstate capacity and skip safety protections.
Questions people ask
Q: Can I take a 20,000 mAh power bank on a plane?
A: Yes. At about 74 Wh it is under the 100 Wh limit, so it is allowed in hand luggage without approval. It cannot go in checked baggage.
Q: Why does my power bank charge my phone fewer times than expected?
A: The rated mAh is measured at the cell's 3.7 volts, and some energy is lost converting it for your phone. Around two thirds of the rated capacity actually reaches the device.
Q: Is a bigger power bank always better?
A: Not for most people. Larger banks are heavier, take longer to recharge and may be restricted on flights. Buy the smallest one that covers a typical day for you.