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.
Maximum 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.
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.