Picking a power bank for EU and Greece-bound travel is a sizing problem, not a capacity problem. The airline limit — 100 watt-hours per pack in the cabin, ideally published UN38.3 — is the regulatory floor; the EU 2-year legal guarantee on conformity is the consumer floor; and the longest window you actually need is the operational floor. Choose the smallest mAh envelope that clears all three, and the rest of the kit finishes itself. The first decision inside that envelope is protocol match. PD 3.1 (USB-C, up to 240 W Extended Power Range) is the right match for the devices EU/GR travel actually demands — MacBook Air, MacBook Pro, iPad Pro, Steam Deck, Nintendo Switch, and USB-C Android flagships. QC 4.0+ (USB-A or USB-C, up to 100 W with PPS) is the right match for Android phones that negotiate PPS and for legacy USB-A accessories you still carry. Pick a pack that lists both protocols rather than a peak wattage without protocol support — a 100 W number that does not speak PD 3.1 will charge a MacBook at 15 W, which is not why you bought a 100 W pack. The second decision is the brick. GaN II is what makes a laptop-class charger sensible as carry-on. GaN transistors switch at higher frequencies with lower switching losses, so the transformer, magnetics and heatsink are smaller for the same wattage. Practically: a 65 W GaN II brick is roughly a third the size of a silicon-brick equivalent, and a 100 W GaN II laptop charger fits the side pocket of a backpack. Lower waste heat also means slower thermal throttling on a sustained laptop charge. Capacity vs weight is the third decision, and it scales linearly. A 10K mAh pocket class sits around 180 g — the right size for a phone over a commute. The 20K–27K mAh travel class lands at 400–600 g and covers a phone plus a tablet across a full day; the 30K–56K mAh power-station class runs 800 g to 1.2 kg and is what you want when the kit includes a laptop. Going past 56K pushes the pack into checked-luggage territory on most airlines — the wrong tradeoff for cabin-bag travel. MacBook and iPhone compatibility is the fourth. MacBook Air (30–45 W) and iPad Pro (30–45 W) negotiate cleanly off any 45 W+ brick. MacBook Pro 13-inch pulls up to 61 W over USB-C PD, so a 65 W GaN II brick keeps it topped up at full speed. The 15-inch and 16-inch MacBook Pro ask for 96–140 W — the GaN II 100 W and the 140 W PD 3.1 power-bank classes are the right match. iPhone 15 Pro and later negotiate 20–27 W over USB-C PD (PPS where supported); older iPhones on Lightning top up at 20 W via a USB-C-to-Lightning cable. Steam Deck and Nintendo Switch pull 15–40 W and are covered by a 45 W+ brick. Match the pack to the plate, not to the mAh. The right EU travel power bank is the smallest pack whose watt-hour envelope clears the airline paperwork, whose protocol stack covers the highest-draw device on the trip, whose GaN efficiency keeps the chassis carry-on-pocketable, and whose wattage hits the device on the title. Pick by the longest window, by the highest-draw device, and by the weight you are willing to carry — and the mAh falls out of the equation.