Wall plugs will have another, electric stove, water heater, another... and if you measure current there, you could get away with one Arduino knowing more or less the status of the house with very little work done in terms of installation (compared to installing on every light switch in the house) and you'd get a lot more valuable data that could enable you to change habits, etc, etc...
Ahhh - an excellent idea! Thank you.
Although, that said, I'm not too bothered about the expense - this sort of thing is like paying National Insurance to me; it's just one of those overheads that has to be taken into account ;-)
I also have a big bag of magnetic switches that I'm intending to fit to all the doors in the house, I'm going to need a way to broadcast to the nextwork the status of each door.