This post follows my other one, but is more focused. Many people are thinking soley about range anxiety when we discuss energy. However, I am concerned about the overall efficiency. Obviously as a larger car it won't be as efficient as a smaller car. It also will have a bigger overhead because of it's range related to a bigger battery. However, add the weight and the extra charging to keep the battery warm and what might be lost overnight and where are we?
I own a Z4 with 115K miles. I got 29 mpg real world, with a mix of everything from lots of stop and go to some good highway miles. If we look only at the cost of energy with 3.60 per gallon (NJ prices) and 16 cents per kwh. I look at a "tank" from the Model S at ~250 miles and that would be 8.6 gallons of gas. So the gas would cost $31. Looking at a straight .16*85 it would cost $13.6 and those numbers look great. Now add roughly 10% for loss during charging and we are at $14.96. The model S is still 1/2 the cost. However, what happens in the cold in terms of wasted energy and the loss of driving range. If we look at 175 as a comfortable range with the heater, the rear defroster, etc. So then we are getting about 70% of the range. Assuming we reduce the gas cost to 70% of full we're at $21.7 vs. 14.96 and that's without warming the battery.
So the Model S still comes out ahead and certainly is much more car, but not as amazing as we would hope in gas cheap (dare I say) NJ and electric expensive.