A sensor board based on a chip like 78m6613 so the complicated RMS pitfalls are simple to overcome. An analog input with a a/d converter doesn't work, you need a rapid series of measurement. The Maxim chip 78m6613, but there are others as well, solves all these problems, you can just ask how much power in watt is being used. Would be great for domotica, energy control, energy awareness.