40,000 mAh at 3.6 V — 144 Wh

Same capacity, other voltages

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.

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.

Nearby capacities

Same voltage, other capacities

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.

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.