Question: how are usb devices powered?
Answer: USB is a bus-powered protocol. This means that in additional to the high-speed signal lines, there is a V+(5V in case of USB) and GND line in the cable. The USB host in any PC can source up to 900mA (USB 3.0) or 500mA (USB 2). If the device needs more current to operate, it will have to be given externally.