Curiously - I have a practical application for this idea. We have a farm, where there is one outbuilding used twice a week for meetings (summer and winter), a workshop that is used by a part time mechanic, a boarding dog kennels for up to six and a house that is split into two flats. The whole lot is powered from a three phase metered supply, and generates one large bill!
Splitting it up and working out who of the four parties actually uses what would allow much fairer charging for services, as the "switch it on and watch the meter" approach will never work! It would also be a big stress relief in my life....
I've considered the idea of trying to measure power usage via inductive means, but have not yet come across anything practical and economical enough for domestic use..... Home built data loggers are second nature to me, but it's an accurate measurement circuit that stops me in my tracks....