Focus on Low Kilowatt-Hours, Not Cost Per GallonWhat if a gallon of gasoline cost $2 in the middle of the night, was free at lunch and set you back $8 in the afternoon? Welcome to the world of electric cars. If you buy one, the cost of a fill-up will largely depend on when and where you recharge it, not to mention the rates your utility company offers.

In the U.S., the cost of electricity varies far more widely than the cost of gasoline, from a kilowatt-hour average of 8 cents in Washington state to 36 cents in Hawaii (a kilowatt-hour (kWh) is the amount of electrical energy consumed when 1,000 watts are used for one hour). Someday soon, you might be able to recharge your electric vehicle (EV) for free during a lunch stop at the mall. But if the EV is going to be filled up at home, your rate could be much higher than the national average of 11 cents that the EPA will put on its fuel economy label for EVs.

Because of the variety of utility rates in the U.S., a 2011 Nissan Leaf that's a bargain to drive in Washington — $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 2011 Leaf's preliminary label is shown here, complete with an erroneous 12-cent per kWh figure in the cost estimate that Nissan says it is correcting). 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.