Puttin' on the Watts

Edmonds.com says, it's not that simple when it comes to figuring the cost of powering your electric vehicle.  Electric rates vary from state to state (and even within states), and some places from time to time;

Because of the variety of utility rates in the U.S., a 2011 Nissan Leaf that's a bargain to drive in Washington [state] — $28.29 for 1,000 miles — is pricey in Hawaii, where those 1,000 miles would cost $97.21. A conventional car getting 36 mpg would make that trip for the same money. For consumers primarily interested in driving an EV to save money, it's critical to know actual electric rates (and the current cost of gasoline, for comparison purposes) instead of relying on national averages.

To figure out the cost of fueling an EV, start with the electric car's energy consumption rate, which is expressed as kWh per 100 miles (kWh/100m). This figure will be listed on the EPA's upcoming EV fuel economy label .... The next figure is your home electric rate, assuming that's the primary charging site. Multiply the kWh/100m figure by the electric rate to get the cost per 100 miles. For instance, the Leaf's kWh/100m figure is 34. If electricity is 11 cents per kWh — the national average — it would cost $3.74 to go 100 miles.

Which is only the cost of the power, and that varies during different (peak v. off-peak) times in a lot of places.  

To evaluate the true cost of the vehicle one would have to know the premium paid for the vehicle (minus the tax advantages), as well as all the maintenance and insurance costs.  Also, don't forget to estimate its resale value.

Then one can work on an estimate of the cost of the aggravation of being unable to use the vehicle while it's recharging. Happy motoring.