If you drive a car that gets 23 MPG (the national average), the yearly savings based on my driving habits and electricity costs are going to be around $3000 PER YEAR.
Here is a snippet from my bill. It has three components. On-peak kWh is the top line. Off-peak kWh is the second line. Demand factor is the third line. Demand factor by definition is the most amount of energy used during peak hours for any 15 minute period. That number, the demand factor, is multipled by a rate that changes seasonally. I can completely discount the demand factor from any costs associated with the Volt. Why? Because I know when we generate the peak rate. It happens when the clothes are being washed, the HVAC is on, and I am cooking on the stove. When I know those moments are going to happen, I simply throw the breaker on the car charger. If I allowed my car to charge during those peak moments, it would likely add about '1' to the demand factor, increasing my bill anywhere from $3.50 to $6 a month. Since I make the decision to disconnect the car during peak moments, my car does not affect the demand factor. All that is left is adding up the on peak hours and off peak hours, and that will average out somewhere around 6 cents per kWh.