Can we save power by using a direct serial connection between the Ardunio and the Pi, rather than the USB cable that requires an extra serial-to-USB converter? We will need a level shifter to convert the Arduino 5Volts to the Pi 3.3 Volts. It definitely helps to have an oscilloscope to see if the serial transmission is going through to the other side:

Works with the Arduino UNO and the Arduno Pro Mini.

The power consumption is between 1.3W and 1.5W.