Stumbled across this project - a home made logger of household power use (think: heavy duty killawatt that monitors the whole house instead of just one plug).
An AVR-based logging wattmeter
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This device monitors household power usage and logs it to an SD card. A simple analog front-end amplifies the signals from voltage and current detectors and an ATmega168 microcontroller computes the power consumption using the formula P=V*I. The voltage and current are each sampled at 9615 Hz so the integration should be fairly accurate even for highly non-sinusoidal loads such as computers or fluorescent bulbs.
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A bit out of my league, but probably not for some others here.
A strip chart showing power usage over time.
Our hot water heater uses over 4kW.