When you consider it – it does hold true yet I don’t know if it is really free, if the power is turned off in a house would it still work? Quite possibly, though to really try it requires taking the time too, not to mention shutting off all other power sources in the home from the switchbox, seems that would make it a more accurate experiment. Even if it is a small amount it seems, could work for the most basic things.
Yet really in my view, it seems by setting up solar panels you could do the same thing at a higher cost, that is have free power from the sun. Yet this offers an interesting thing to ponder, and will have to try it. Will you try it to? See if we can get some comments here about your experiences with this idea.