Gabriel Miller has published a simple way of monitoring rotary-disc electricity power meters in order to measure and analyse his household power usage. The method is simple - a laser beam is pointed at the rotary meter disc, which has a small hole near the perimeter. Once every revolution the laser light will shine through this hole and to a photoresistor. Thus the Arduino can then track the number of kWh used and so forth. Please note that working around the mains supply can be dangerous, and your electricity distributor may not be keen on people doing this. However it's a good read and an interesting Arduino-based project.
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