I got inspired to design a low cost energy meter after reading this post. Here is my design. I have a working prototype, the success of which I have to attribute to the people here and their ideas and comments.
One important aspect of this design is that the measurements are not intended to be NIST traceable, only a respectable approximation. I have a section dedicated to sources of error which is long and certainly incomplete.
Speaking of comments, I would certainly appreciate them! Questions and advice, too.