[johladam]

I believe that one way to measure the current for relatively cheap would be to break the neutral line and throw a current-sensing resistor in it. It would create a small voltage drop across the resistor. If you know the voltage drop and the resistor value, it should be pretty trivial to determine the current through the neutral line. Equation to do that would be:

lineCurrent = (Voltage of the current-sense resistor) / (resistance of the current-sense resistor)

Considering a current sense resistor is fairly cheap (http://octopart.com/13fr200e-ohmite-988758) it shouldn't be terribly expensive to do. You should be able to throw that between a Vin- and Vin+ on a linear optoisolator and send the values back to a microcontroller to be reported.

I'm not an EE though, so grain of salt.

[lhl]

Yeah, in theory you could build your own but having something UL rated is an advantage. AC is a lot more dangerous than DC, so you should really know what you're doing before mucking around.

There are a few more components for building a basic board: https://instruct1.cit.cornell.edu/courses/ee476/FinalProject... http://enerjar.net/Hardware.html

Building a more accurate power meter is pretty involved: http://coolarduino.wordpress.com/2013/02/08/power-meter/ http://coolarduino.wordpress.com/2013/02/27/power-energy-met...