If you look at the data sheet download on MikeM's efergy link, it says the meter has an accuracy of <8%!
Now that says it is less than 8% accurate. I expect they mean it is better than 92% accurate. Even so that is not very accurate. But can you get any better with a clip on device and is it worth the money?
I still have a meter with the spinning disc (200 revs = 1 kWh) and find that it is easy to calculate individual loads - and accurate - even for low, say 20 watt loads. Just have to make sure that the background use stays steady (i.e. nothing else switches on or off) during the measuring time for the load under test. Over time (approx 1 month) I have taken enough readings to know just how much I'm using and what is using it. Put it all in a spreadsheet and then compared it over a year to the total recorded on the meter. Result - within a couple of %. So for me logging is not an issue. But come the day when the leccy co decides I need a new meter then I will have to look for another way of measuring the consumption.
