Step 2: Electricity usage sensor unit
Of course you could remove the LEDand direclty grab the pulse from there but as I have a great respect for that box I didn´t dare to dismantle it. Instead I decided on placing a light sensor on top of the LED and get the pulses this way.
Without properly studing light sensors I first ordered some "photo cells" but quickly realized they cant pick up a quick pulse. Instead I got some Photo Reflective Sensors that have a light and IR sensor. With this sensor there was no problem to catch the pulses. Of course I had to cover the area around no to let any additional light in.
How to count the pulses? First I wanted to do this just with the XBee but as I couldn´t figure out a way to easily do it I put a Arduino to provide some logic. The sensor is attached to an analog port and the Arduino does a count++ every time it senses a pulse. A counter on the Arduino counts to 60 seconds and then sends the count over XBee to a coordinator. The Arduino is actually a clone by Seeeduino.
Arduino sketch and XBee config attached.