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...