Great, thanks for the clarifications @Sparkman !
Being able to measure power would of course be a nice bonus! I'll think about it. I have another question (of course... ).
The Nano and Pro mini have only 6 analogue inputs, I would be having need for 8... Would I need to use two arduinos or is there any other way?
Have you thought of sticking a hall effect sensor to the outside of the relay. Or in the magnetic flux.