I have owned my roadster for six months now, and I have enough data on energy use to calculate a rough estimate of energy cost / year and miles / dollar. For this discussion I am assuming that the car is driven around 10,000 miles per year; I have been driving the car only slightly less than that on a yearly basis. The numbers listed here are current 2009 rates from PG&E, which serves most of Northern California.
My calculations conclude for a solar powered home such as mine, the cost per year is from $0 to $270 for 10,000 miles. With solar installed and exactly enough for just the house, the electricity cost for the car would be $270 for the incremental off-peak rate of $.09 / kWh available for solar powered homes. The energy cost would be technically free if you are generating enough solar energy for both the house and the car. Of course, this cost does not consider the cost of installing the solar panels or any lost opportunity cost of the original investment. When I installed solar in my house, I had only lived here for a short time so the estimate of the number of panels was a little inaccurate. For the first two years, I produced more energy than I used and gave PG&E some free energy. In 2008, I gave PG&E free energy worth $155. So I would estimate that the cost for driving the car 10,000 miles for me will be around $115 per year ($270-$155). The worst-case scenario miles per dollar rating for a solar home is 37 miles per dollar.
For a non-solar powered home, the energy rates vary a lot depending upon the amount of energy used. As you use more energy, the costs go up significantly. The rates start at $.115 / kWh and rise up to $.44 / kWh with the average listed as $.176 / kWh. For charging a car, the correct calculation would be based upon the incremental energy usage depending upon your current usage. For a typical 10,000-mile year, your costs could range from $345 to $1320 per year strongly dependent upon you current electrical consumption. Without solar, the miles per dollar in Northern California would range from 29 to 8 miles per dollar.
The calculation methodology of course is important. Unfortunately, I have no method to very accurately determine the energy use. The car screens only report two energy use numbers: a monthly number and a trip mileage number. Neither is particularly useful. The monthly number resets with the month, requiring you to take a note of this number at the end of every month. The trip odometer appears to reset itself after 999 miles.
In order to calculate my energy costs, I used some of the data from the trip summary. I also used the data from my PG&E energy statements. I can look at these reports to determine the additional energy use during the months I have owned the car using two different methods. One a strict year-to-year comparison of total energy during the last six months. Then I also looked at the difference for the summer off-peak energy use difference. All of my data ranged from 3.68 m / kWh to 2.9 m / kWh. I used a rough average of 3.3 miles / kWh in order to calculate the costs. Of course all of this information depends upon your driving style and your local energy rates.