Power consumption

USB 2.0 Specification, Section 7.1.2 & 7.1.5
USB 3.0 Specification, Section
ECN USB 2.0 Connect Timing Update

In order to measure the power distribution of a USB device, the average current is measured during unconfigured, configured, active and suspend state with a digital multimeter and fixture to measure the Vbus current . The circumstances for measuring the average current are dependent on the speed of the device and the power mode the device is in (e.g. self powered, bus powered or battery charging capabilities). All High Speed measurements are performed by connecting the device under test after one self-powered High Speed hub. A Full Speed self-powered hub is connected to the first High Speed hub in order to force a High Speed device under test to enter its Full Speed mode. Beware that according to USB 2.0 Spec Vbus should stay between 4.4V and 5.5V, make sure you measure it in worst case condition.

Speed conditions
For a High Speed device the average current is measured in High Speed and Full Speed mode. For a Full Speed device the average current is measured in Full Speed mode. For a Low Speed device the average current is measured in Low Speed mode. A Super Speed device is measured in Super Speed and High Speed mode.

Power conditions
A device should also be measured using the supported power mode in the following circumstances:
1) When a device is only capable of operating in self-powered mode, all measurements are per formed in self-powered mode. This means that the device is unable to enumerate without being connected to an external power.
2) When a device is capable in operating in bus-powered mode, all measurements are performed in bus powered mode even when the device claims to be self-powered (in its device descriptor).
3) When a device has battery-charging capabilities over USB, the power measurements are performed in worst case scenario, most probably this is when the product has a dead battery.

Unconfigured current
The USB 2.0 device under test is set in unconfigured state by using the tool USB20CV. The USB 2.0 unconfigured current must remain below 100mA at all times. For a USB 3.0 device the unconfigured state can be force by using USB30CV. A USB 3.0 device may consume 150mA in unconfigures state. The unconfigured current is measured in the above conditions.

Configured current
The device under test is set in configured state by using the tool for USB2.0 USB20CV and for USB3.0  USB30CV. In this state the configured current is measured in the above conditions. The configured current must remain below the value defined in the bMaxPower field of the descriptor and may never exceed the maximum allowed 500mA for a USB 2.0 device and 900mA for a USB 3.0 device. However there are devices that may consume more than the 500mA or 900mA when following the battery charging specifications.  

Active current (U0)
The device under test is operating correctly and during operation, the device current is measured using the above conditions. The active current must remain below the value defined in the bMaxPower field of the descriptor. Make sure that the current is measured in worst-case power consumption mode. For USB 3.0 Super speed this is U0 mode.

Suspend current (U3)
After the device under test is correctly enumerated the host system is placed in Suspend S3 Mode. When the device supports remote wakeup, the remote wakeup feature must be enabled during measurement. The USB 2.0 spec allows a maximum suspend current of 2.5mA. In general the suspend current is the most difficult requirement to meet. For USB 3.0 Super Speed this state is called U3.
Note that bus powered compound device (internal USB hub with embedded devices) may consume 2.5mA for the internal hub plus 2.5mA for each compounded device. (maximum 4 ports = 12.5mA)

Dead Battery current
If the USB device have battery charging capabilities the device must undergo the current measurements with dead battery.  Beware with the ECN USB 2.0 Connect Timing Update taken into account it's allowed for battery charging devices to consume up to 500mA and not 100mA as mentioned. All battery powered devices must undergo this dead battery test, it not matter if the product is according to the Battery Charging 1.2 Specification or not.  Device that not charge batteries may only consume 2.5mA as defined in the next topic Suspend current Powered state.

Suspend current Powered state
This measurment is not the regular suspend current measurement as described above.Do not forget this measurement!
Peripherals are required to support the suspend state whenever VBus is powered, even if bus reset has not occurred. For this test you just need a A-meter a high speed hub and USB20CV. After doing the power measurements in USBCV - unconfigured and configured state- do not switch off USBCV just deatch and reattach the device and measure the current. The current should stay below 2.5mA and below 100mA for devices that do battery charging over USB. Here more details

powered suspend

USB 3.0 Super Speed U1/U2 low power state
Beside U0 and U3 for super speed that are explained above, the USB 3.0 specification also defines two other low power states U1 and U2.
With USB30CV you are able to force the device into U1/U2/U3 power states. The power consumption for U1 should be lower than U0 and U2 should be lower than U1. The USB 3.0 specification not mentions a value on how much lower the current should be.