Do people with electric cars only charge once per week? I concede There could be some demand response if people could know that they can charge for less every Sunday night. But as you say when you are empty you are a price taker.

I live near Vancouver which features the most expensive gas in North America, and decent public transportation, and I believe traffic is the only thing that keeps people from driving more, not cost. So demand seems inflexible over the long term, and with electric cars I think you have even less flexibility to choose your time and place to charge, unless charging becomes ubiquitously available.