This is what the actual power consumption of the AC phase, the fridge is connected to, looks like:
You can clearly see, that there is a regular pattern in that graph. There seem to be two different devices consuming power: One is active for about an hour consuming roughly 100W and then pauses for ca. 2 hours. The other device takes about 40 W and turns on for 40 minutes, 14 times a day. Both powers add up when both devices are running simultaneously, that explains the humps on the curve.
My guess was, that the 100 W device is the old fridge and the 40 W device the pretty up-to-date freezer. So - how to find out? One could simply plug out both devices - but that would defrosts the groceries inside the freezer and would be much too simple :)
So i build a wireless sensor for embedding in the fridge :) This is what i came up with:
It is a small plastic box containing a Jeenode equipped with a 433 MHz RFM12 transceive, with the antenna folded inside the box. It is attached to a 3xAA battery box and to a DS1820 one-wire temperature sensor via a 3.5 mm audio jack.
Most of the time it is in deep power-down mode, but once every minutes it wakes up and transmits the temperature reading out of the fridge to the 433 MHz accesspoint. I found it quite impressive that it manages to establish a bidirectional communication out of what is basically a faraday cage! This is what the reading from inside the fridge looks like:
So, that was fun - next!