9.2.5.1 Power Budgeting
USB bus power is a limited resource. During device enumeration, a host evaluates a device's power
requirements. If the power requirements of a particular configuration exceed the power available to the
device, Host software shall not select that configuration.
USB devices shall limit the power they consume from VBUS to one unit load or less until configured.
Suspended devices, whether configured or not, shall limit their bus power consumption as defined in
Chapter 7. Depending on the power capabilities of the port to which the device is attached, a USB device
may be able to draw up to five unit loads from VBUS after configuration.