100 power bank cells — planes count watt-hours, not milliamp-hours

20,000 mAh

• 3.6 V72 Wh · carry on freely
• 3.7 V74 Wh · carry on freely
• 3.85 V77 Wh · carry on freely
• 5 V (USB)100 Wh · carry on freely
• 11.1 V222 Wh · not allowed

Why the rule is in watt-hours

How big a fire can get is set by the energy inside, not the charge. Milliamp-hours measure charge alone; multiply by voltage and you get energy, measured in watt-hours. A pack that stacks cells in series to raise voltage carries proportionally more energy at the same mAh — which is exactly why the rule reads in watt-hours.

The thresholds are 100 Wh and 160 Wh

Up to 100 Wh you carry it on without asking; from 100 to 160 Wh you need the airline’s approval; above 160 Wh it may not travel on a passenger aircraft at all. In every case it must ride in the **cabin**, never in checked baggage — a fire in the hold is one nobody can reach. Individual airlines may set stricter limits.

Counted at 5 V, the number shrinks

The printed mAh is measured at the cell’s voltage. Output comes out at 5 V, so counting the same energy at 5 V gives a smaller number: 20,000 mAh at 3.7 V is 14,800 mAh at 5 V. Much of the feeling that a bank “delivers less than the label” comes from this conversion; the rest is the loss in stepping the voltage up.

Higher-voltage packs hit the limit sooner

At 3.7 V you stay under 100 Wh up to 27,000 mAh; an 11.1 V pack crosses the line past about 9,000 mAh. Large laptop and camera packs sit in that range. If the case prints a Wh figure, use it; if not, multiply mAh by voltage yourself — at the gate the answer they want is in watt-hours.

These figures follow the label

A cell’s real voltage swings between about 3.0 and 4.2 V as it charges and drains; the nominal figure just stands for the middle. Printed mAh is also sometimes generous relative to the cells inside. If your pack sits near a threshold, go by the manufacturer’s own Wh marking.

3.6 VLargest free capacity at this voltage 27,778 mAh

3.7 VLargest free capacity at this voltage 27,027 mAh

3.85 VLargest free capacity at this voltage 25,974 mAh

5 V (USB)Largest free capacity at this voltage 20,000 mAh

11.1 VLargest free capacity at this voltage 9,009 mAh

How to read this

• Watt-hours = mAh × volts ÷ 1000. 20,000 mAh × 3.7 V = 74 Wh.
• Up to 100 Wh is free, 100–160 Wh needs airline approval, above 160 Wh is barred.
• Either way it goes in the cabin, never in checked baggage.
• Counted at 5 V the figure comes out smaller than the label.