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.
OSD
I have had only one problem with my project and that is the power supply I used was rated for 220V. This turned out to be a problem on my side of the pond as the USA is 240V. After frying two, I found a 240VAC to 5VDC PSU which has been working fine for a month now
Just for your enlightenment or horror, our maximum legal voltage in the UK (which can be exceeded on occasions in some places) is 254 V. (i.e. the old 240 V + 6% or the “harmonised” European voltage of 230 V + 10%.)
The obligatory Wikipedia link: Mains electricity by country - Wikipedia