What is the voltage of a USB?

Clients have to accept 4.75 to 5.25V and must be able to send configuration data as low as 4.4V. Hubs are allowed to drop as much as 0.35V.

By itself, a USB plug does not have to supply more than 100mA; however up to 500mA can be negotiated between device and host. Some USB hosts simply offer up 500mA from the get-go.

USB power supplies short the two data lines (white and green) which allows the device to probe for the fact that they're power supplies (i.e. that they provide the full 500mA without negotiation and that they won't respond to data).