The Open Energy Monitor uses an AC transformer as a voltage sensor, but the choice of this transformer can influence the accuracy of your monitor.
Jean Claude from JeeLabs (brilliant engineer BTW) investigated the difference between a bad and a good AC transformers and here are the results in images:
1) Bad quality transformer:
2)Better quality transformer:
You can read his complete post on his blog: JeeLabs Daily Weblog.To engage in discussion regarding this post, please post on our Community Forum.