Powered USB, also known as Retail USB, USB Plus Power, and USB + Power, is an addition to the Universal Serial Bus standard that allows for higher-power devices to obtain power through their USB host instead of requiring an independent power supply or external AC adapter. It is mostly used in point-of-sale equipment, such as receipt printers and barcode readers.
Powered USB, as a proprietary variant of USB, was developed by IBM, NCR, and FCI/Berg but is not endorsed by the USB Implementers Forum (USB-IF). IBM, who owns the patents to Powered USB, charges a licensing fee for its use.
Powered USB uses a more complex connector than standard USB, maintaining the standard USB interface for data communications and adding a second connector for power. Physically, it is essentially two connectors stacked such that the bottom connector accepts a standard USB plug and the top connector takes a power plug.
The implementation allows a choice of three different voltages, providing power at 5 V (30 W), 12 V (72 W), or 24 V (144 W). The three voltages are able to operate at up to 6 A. USB 1.0 and 2.0 supplies 5 V at up to 0.5 A (2.5 W). USB 3.0 supplies 5 V at up to 0.9 A (4.5 W).
As each Powered USB plug provides one of three voltages, the plugs come keyed in three versions so they will only accept connections from devices requiring that version’s voltage.
Extending USB's power capability is a response to Power over Ethernet which has more flexible dynamic power negotiation capabilities up above 48 V DC and up to about one amp, and whose maximum bandwidth is potentially greater than USB 3.0's once the 10 Gbit/s Ethernet standard is supported. The combination of USB 3.0 and a 6 A limit at the most common (5, 12, 24 V DC) voltages permits support of a wider variety of devices, although powered Ethernet can support them at longer cable distances, power loss in small DC voltages are very significant over even a few meters, so it is unlikely that powered USB can compete over longer distances effectively.