I think a large part of it comes from charging the vehicle at night when there is excess capacity in the utility's generating system and the cost of electricity is low. Then it may be sold back when the car is parked at work and the grid needs peak power which is expensive.
Looking farther ahead, it could come from fuel cells or other ideas which right now are not economically realistic.
Re post #21. The electric company is going to buy back it’s own juice at a loss instead of buying it’s own batteries?