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The EnerJar is an easy-to-build device that accurately measures the power draw of electrical appliances. The EnerJar was the winner of the Greener Gadgets design competition. The design is still being optimized, but preliminary schematics and source code can be found on this site by following the "Hardware" and "Software" links to the left.

http://www.enerjar.net/

// Erik (binkey.nl)

Now that's a nice hobby project! I would probably opt for a plastic lid though

Looking at the source code, it seems to do a pretty accurate measurement: Voltage and current are measured 1000 times per seconde, each voltage/current pair is multiplied and put it a moving-average-array. So even those capacitive power supplies and the hit-and-miss modes used by power supplies in modern TV sets can be measured accurately.

The hardware is even capable of measuring cos-phi; That would be a nice addition.