What the CAA actually allows
The rule that governs every UK departure is the Civil Aviation Authority's, and it's simpler than most airline pages make it sound. A power bank rated up to 100 watt-hours is fine in the cabin: the CAA lists lithium-ion banks "up to 100 Wh rating ... allowed without operator approval". A larger lithium-ion bank, "greater than 100Wh ... but not exceeding 160Wh", is still allowed, but only with "approval from the operator", which means asking your airline first. Above 160Wh, a bank is not permitted in passenger baggage at all. Either way the regulator is firm on where it travels: a power bank "must only be carried as carry-on baggage", never in the hold.
Working out your bank's watt-hours
Watt-hours, not milliamp-hours, are what the rule is written in, so the number you need may not be the big one printed on the front of the pack. Most banks show the Wh figure somewhere on the casing; if yours lists only mAh, the working estimate is its mAh multiplied by 3.7 and divided by 1,000, because the cells inside run at roughly 3.7 volts. That puts a 10,000mAh bank at about 37Wh and a 20,000mAh bank at about 74Wh, both comfortably under the 100Wh line. You don't reach 100Wh until somewhere around 27,000mAh, so for the great majority of travellers the watt-hour limit never bites: the pack already in your bag clears it.
What changed on 27 March 2026
The CAA tightened the passenger rules on 27 March 2026, and three of the changes are the ones that catch people out. First, there's now a hard count: a "maximum of two power banks per person". Second, you can no longer top the bank up in flight, because it "must not be recharged on board the aircraft", so the in-seat USB port is off limits for it. Third, the CAA says a bank "should not be used to charge other portable electronic devices on board the aircraft" either, which is the part frequent flyers find most surprising, since charging a phone from a pocket bank at cruise used to be routine. The practical answer is to board with everything already charged.
Never in the hold, and protect the terminals
The cabin-only rule exists because a battery fault is something the crew can see and reach at your seat, and can't in the hold. It's also why, if your cabin bag has to go into the hold at the last minute, the bank has to come out and travel with you in the cabin. For loose spare batteries the CAA adds one more step: each must be "individually protected so as to prevent short circuits", which it suggests doing "by placement in original retail packaging or by otherwise insulating terminals", for instance taping over the contacts or slipping each cell into its own bag. A sealed power bank keeps its cells protected already; a bare spare battery doesn't.
Your airline can still say no
The CAA sets the floor, not the ceiling, and its own wording hands the rest to the carrier: a bank over 100Wh flies only with "approval from the operator". In practice the airline you're booked with (for instance easyJet, Ryanair, British Airways or Jet2) publishes its own policy, and some are stricter than the regulator. So if your bank is a large, high-capacity model, or you plan to carry more than one, read your carrier's own page before you pack rather than trusting a general guide. For choosing a travel bank that stays on the right side of all this, our power banks for long flights guide has the in-seat picks, and the 20,000mAh power bank guide is the everyday size that clears the 100Wh line without you doing any maths.
FAQ
Q: Can I take a power bank in my hand luggage? A: Yes, and only there. The CAA says a power bank "must only be carried as carry-on baggage", so it goes in the cabin and never in the hold. If your cabin bag has to go in the hold, take the bank out first and keep it on you.
Q: What size power bank can I take on a plane? A: Up to 100Wh needs no permission. A bank between 100Wh and 160Wh is allowed only with your airline's approval, and anything over 160Wh is not permitted in passenger baggage. Most 10,000mAh and 20,000mAh banks are well under 100Wh, roughly mAh times 3.7 divided by 1,000, so they're fine.
Q: How many power banks can I bring? A: Since 27 March 2026 the CAA sets a "maximum of two power banks per person". If you need more capacity than two banks hold, that's a conversation to have with your airline before you fly.
Q: Can I charge my phone from a power bank during the flight? A: The CAA says a bank "should not be used to charge other portable electronic devices on board the aircraft", and it "must not be recharged" in flight either. Charge everything before you board, and save the bank for the ground.