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...
Although, that said, I'm not too bothered about the expense
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