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Definition: USB power

The USB-IF has defined power ratings with each USB version as well as a Battery Charging specification for power adapters. Over the years, the power capability of USB increased substantially from the original 2.5 watts to 100 (see USB Power Delivery). See also poweredUSB.
  Version     Volts Amps    Watts

  USB 2.0      5v   .5A     2.5w

  USB 3.0      5v   .9A     4.5w

  Battery Charging
  BC 1.2       5v  1.5A     7.5w
  (see below)

  Power Delivery over USB-C cables
  PD           5v    3A    15w
  PD           9v    3A    27w
  PD          15v    3A    45w
  PD          20v    5A   100w
  See USB power delivery.

USB Battery Charging
When USB Battery Charging 1.2 was introduced in 2010, the following terms were coined to define the USB port's functionality.

Standard Downstream Port (SDP) - Data transfer only.

Dedicated Charging Port (DCP) - The port in a power adapter for charging only; no data transfer.

Charging Downstream Port (CDP) - A port that is capable of transferring data and charging the device at the same time. Also called a "charge and sync" port.