I have fitted the emonTx in its case, improved the accuracy of the calculation of total power usage, and deployed it:
I will need to monitor over some time to see how accurate it is.
It will be more interesting when I correlate it with measurements from individual appliances, and when I have enough data to compare it with the data from my electricity supplier.
I already know roughly where all my electricity usage is, and I know of various ways that I can improve it. For example, I have mainly halogen lights in my house, and I have replaced most of them with LCD bulbs, apart from 10 50w halogen downlights in the kitchen. These are 12v MR16 bulbs and each has its own transformer. I am monitoring when LCD replacements for these become bright enough, cheap enough, and will work with the existing transformers.