Oh Wow... I had sketched something like that a few months ago. I thought it would be cool if the device could work out the Kilowatt/Hr after putting in your provider's rate and it would display (in currency) the cost so far used since the appliance has been switched on (i.e £0.23).
They're cheap at under £20 GBP incl. P&P. I'm going to buy loads.
If they're 13-Amp I suppose you could put a whole multi-socket onto that one plug to see what your combined printer, scanner, monitor & Amiga setup uses!
Somehow I don't think a Cyberstorm PPC would be a good companion to a solar powered house in the UK.
;-)