know how many watts a USB device is consuming
System Information shows how much current devices tell they require at most:



The voltage for USB (including 2.0 and 3.0) is about 5 V, so 98 mA is about 0.49 W. For some devices the required current is just shown as 500 mA (the USB 2.0 maximum), and I don't know how to see how much energy devices actually consume.