http://www.anandtech.com/show/10163/usb-flash-drives-power-consumption-measurement-using-plugables-usbctkey
Coming back to our original intent of measuring the power consumed by USB peripherals such as flash drives, it is obvious that we need to be measuring the voltage and current on the VBUS line. This aspect has suddenly become something to care about because of the number of mobile devices supporting 'OTG'. Not all mobile devices are able to provide the 900 mA output current to 'bus-power' attached devices. Note that the initial specification for USB only had provisions for 0.75 watts of power – 150 mA at 5 V. USB 2.0 bumped that to 500 mA, or 2.5 watts, and USB 3.0 specified 900 mA at 5 V, or 4.5 watts. Understanding the power consumption profile of a flash drive is essential to determine whether it can be safely used with a particular mobile device. It can also help in understanding the impact of usage on the battery life of a mobile device. From a technical viewpoint, it also gives insight into the efficiency of the USB peripheral itself.
If it runs at 3.3v, it'll chew your battery to bits
Pretty cool though