Generally, an USB connection provides five volts on a single wire that a USB device can draw power from. This setup cannot provide more then 5.25 volts and no less then 4.35 volts.
When a device is connected, it is allocated 100mA of power. After initialization, the device can request more power in 100mA units up to a maximum of 500mA. Most ports are able to provide the maximum amount of power. If the device requests more power then the port can provide, it will not be allowed to operate until the user rearranges the network situation to provide more power.
When a bus-powered hub is used, the maximum power provided to each device is 400mA. This limits the hub to no more then four users. If a device needs more power then can be provided, it must utilize its own power supply to compensate for the amount of power needed. The operating system keeps track of the power requirements and specifications, and it may alert the user if there is a power supply problem.