Will this power bank fly?

The number printed on a power bank is in milliamp-hours. Airlines, however, think in watt-hours — and so does your phone, in a roundabout way. Type two numbers below and get the answer both of them care about.

Why 3.7 volts?

A power bank's rating describes the lithium cells inside, not the 5 V (or 9 V, or 20 V) that comes out of the USB port. Most lithium-ion cells have a nominal voltage of about 3.6–3.7 V, which is why that is the default here. Energy is charge multiplied by voltage, so 20,000 mAh × 3.7 V ÷ 1000 = 74 Wh. The ampere-hour on its own says nothing about energy until you know the voltage it was measured at.

Why the phone-charge count is lower than you expect

Stepping 3.7 V up to 5 V or 9 V, heat in the cable, and the phone's own charging circuit all cost energy. Measured efficiency usually lands between 60% and 75%, so a 20,000 mAh pack rarely delivers four full charges to a 4,500 mAh phone — closer to three. Battery University covers why lithium cells also lose usable capacity as they age, which pushes the real number down further over time. If you are still deciding which size to buy, this breakdown of a power bank 20000mah versus the 5,000 and 10,000 mAh tiers walks through the same trade-off with weight and charging times.

The airline limits

Spare lithium batteries, power banks included, must travel in the cabin. These thresholds come from the IATA dangerous goods guidance and are echoed by the FAA PackSafe rules. Individual airlines can be stricter, and several now ask that power banks stay out of overhead bins, so check your carrier before you leave.