Dan made an AVR-based logging wattmeter… He writes –
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. A graphical LCD shows the power usage as a strip chart and can also act as an oscilloscope to display the voltage and current waveforms. The current is amplified in three stages (1x, 10x, and 100x) so that different gains can be used giving accurate readings for both high and low power usage.
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If you like this idea but don’t feel like running wires out of your breaker panel, you might check out TED, or “The Energy Detective”: http://www.theenergydetective.com
It has clamp-on current transformers and a measurement unit that fits inside the breaker panel, samples and integrates voltage and current, then sends power readings out over a 2400 baud powerline modem. Their kit includes a little LCD panel that receives current power usage and calculates monthly totals and such.
But if you want a standalone datalogger, their protocol is pretty easy to interface an AVR to. I have some open source schematics and firmware for doing so:
http://scanwidget.livejournal.com/36469.html
My receiver just spits out ASCII text on a serial port, and I have a low-power PC recording it to disk and making charts with rrdtool. But it should be easy to hook it up to another microcontroller with an SD card, for self-contained power logging.