Everything that charges from a USB port runs from 5 volts.
The amp-hours are, for change, quoted for the battery voltage, which is about 3.7 volts (3 to 4.2V, with the plateau at 3.7).
…therefore the energy storage is about 74 watt-hours.
At 5 volts, with 85% efficiency of the boost converter inside, the capacity is 12.6 amp-hours.
