Math: Let us assume that you are a foster parent in France, take care of nine kids, and thus you drive a monster of a people mover with a onehundred kwh battery, and you go through one charge per week. Top it off to full each night during generic offpeak hours, and you are out 8.75 euros /week. Most of which is, in fact, taxes. Which you cannot avoid by messing about trying to time the electricity market. So, at most, if you watch the weather reports like a hawk, and plan your driving very carefully around them, you save.. 2 euros per week. 104 euros per year. And this assumes that you are driving the electric equivalent of a SUV. For a smaller car, it is going to be half that.
Nobody is going to actually do this - it is far too much work for far too little return - Certainly it will not be common enough to affect overall demand at all. The day/night demandshifting works because it is automated and transparent to the consumer - Plug in at night, charged by dawn.