Cheap Point of Use Electric Power Monitor

Here's the correct link for "Simple Digital AC Wattmeter".

http://electronicdesign.com/lighting/simple-digital-ac-wattm...

That's a very neat circuit. It's a very good wattmeter. It measures instantaneous voltage and current, multiplies them in an analog multiplier, integrates the result, and produces a 5V digital pulse for each joule used. So it will measure correct wattage for devices with non-standard power factors, and work with power sources that are not pure sinusoids. Most such devices just measure current and assume voltage.

This is the kind of wattmeter you need to measure power consumption of things like the E-Cat cold fusion device, with its "proprietary waveform" that's probably being mis-measured. That would be a useful bench instrument to have around. For the application proposed, it's unusually complex, but not a bad design. Bob Pease, who used to write an analog column for Electronic Design, would have liked it.

The usual solution today is a Hall-effect current sensor, like this: https://www.sparkfun.com/products/8882 It only measures current, but it's easy to use and electrically isolated from the high voltage supply. It doesn't require a high-current low-resistance current-sensing resistor (0.002 ohm) in the high voltage side as the one above does.