Hello Ladyada and anyone else interested. I have recently been building various energy monitoring / power metering bits and pieces both for mains AC energy use and wind turbine monitoring. I have so far been concentrating on the measurement side: Building circuits to do the job of the killawatt meter in your tweet-a-watt project. Ive had a some luck with a simple circuit using a clip on CT sensor connected up to an Arduino that may be of use to you in the tweet-a-watt project.
I'm measuring current using an efergy elite CT sensor and then "synthesising" the voltage waveform using the current waveform measurements. You just enter the rms Voltage and frequency, the arduino program creates a sine wave from these values that is aligned with the current waveform: the peak of the synthesised voltage waveform is made to align with the peak of the current waveform. Im not sure if this is the best way to do this but I seem to be getting good results the real power, and power factor measurements using the CT method seem to agree quite well with measurements from a plug-in power meter that measures both the voltage and current directly.
I have written a page about it including circuit schematics and arduino sketch here: http://openenergymonitor.org/emon/node/28
Here's a picture of the electronics:
A picture of the output graphed in KST.http://openenergymonitor.org/emon/sites/default/files/Output.png
Results comparison between the CT method and a plug in power meter.http://openenergymonitor.org/emon/sites/default/files/Results.ods
My next step is to follow your guide on the xbee's and posting to twitter, I have bought two xbees from you and built your adapters. Thanks for the fast delivery and great build documentation!
Hope the above can be of use