Over 100Wh: the airline's approval
Between 100Wh and 160Wh, the CAA's "approval from the operator" means your airline's say-so. Ask the airline you're booked with (easyJet, Ryanair, British Airways or Jet2, for instance) before you pack a bank that size. A bank over 160Wh falls outside both of the CAA's allowances.
Working out your bank's watt-hours
The rule is written in watt-hours, not milliamp-hours, so the big number on the front of the pack may not be the one you need. If your bank lists only mAh, multiply by 3.7 and divide by 1,000, because lithium-ion cells run at roughly 3.7 volts. That puts a 10,000mAh bank at about 37Wh and a 20,000mAh bank at about 74Wh. You don't reach 100Wh until around 27,000mAh.
What changed on 27 March 2026
The 27 March 2026 changes also cover use in flight. Besides the recharging ban, the CAA says a bank "should not be used to charge other portable electronic devices on board the aircraft", so the bank shouldn't top up your phone at cruise, and the in-seat USB port mustn't top up the bank. Board with the phone and the bank already charged.
Never in the hold, and protect the terminals
If your cabin bag has to go into the hold at boarding, take the bank out and keep it with you. For loose spare batteries the CAA adds one more step: each must be "individually protected so as to prevent short circuits", either "by placement in original retail packaging" or "by taping over exposed terminals or placing each battery in a separate plastic bag or protective pouch".
Choosing a bank for the trip
Our power banks for long flights guide compares three packs for a long-haul trip, and the 20,000mAh power bank guide covers the everyday size that clears the 100Wh line without any maths.
FAQ
Q: What is the limit for a lithium-metal power bank? A: The CAA's allowance without airline approval is "2 g lithium content" for lithium-metal, in place of the 100Wh rating it uses for lithium-ion banks.