Even if the expensive infrastructure was put in place to allow electric car batteries to be used as a peak load reservoir, utilities would still need to maintain traditional on demand resources, such as natural gas fired power plants. Using a large number of cars is not dependable, utilities need on demand dependability. What happens if during peak consumption in the evening winter months more people stay out late (such as on New Years Eve) and most people park in places where there's no way to plug in? This would only add to overall infrastructure costs, adding a huge bidirectional grid in addition to traditional power sources that must be maintained year round for on demand use.