What it really costs to fill up a plug-in car

NEW YORK (CNNMoney.com) -- Even if they aren't always terribly accurate, EPA fuel economy estimates have at least made it easy to compare the gas mileage of one car to another. But now that plug-in cars are entering the market, things are about to get much more complicated.

How do you compare the fuel efficiency of a car that runs on gasoline to one that plugs into an outlet?

Last week, Nissan and General Motors each released preliminary EPA fuel economy labels for their plug-in models. The labels, particularly for the Chevrolet Volt, which runs on both plug-in electricity and gasoline, seem sure to provide at least as much confusion as assistance to potential car buyers.

We'll try to sort out some of the trickier issues.

Q: The EPA's label seems to say that the Nissan Leaf, which uses no gas, gets 99 miles per gallon. How is that possible?

A: Actually, the Nissan Leaf gets 99 miles per "gallon equivalent," or MPGe. That means the amount of electricity the Leaf uses to drive 99 miles is equivalent to the amount of energy in a gallon of gasoline. (If you really want to know, there are 33.7 kilowatt-hours in a gallon of gasoline.)

The Chevrolet Volt, by comparison, gets 93 MPGe when using electricity from its battery.

Q: If it's not burning gasoline, why do I care about MPGe?

A: You probably don't. The concept of MPGe was created as a way to compare the efficiency of an electric car to that of a gasoline-powered car which, the Environmental Protection Agency says, consumers indicated was important to them.

But, since electric cars don't burn gasoline, that comparison is of limited use. For those concerned about the environment and global warming, it does at least show which vehicle uses fuel -- gasoline vs. whatever is being used to generate the electricity -- more efficiently. A better comparison for consumers might be the "cost per year" to fuel the car. That's also shown on the EPA labels of all cars.

Q: So what's the fuel economy for the Chevrolet Volt?

The question is, which fuel economy are you talking about? The Volt gets 93 MPGe when running on plug-in electric power. The EPA estimates it should be able to do that for about 35 miles on a full charge.

After that, it's estimated to get about 37 miles per gallon when running on gasoline. The sticker does present a "combined" figure of 60 MPGe, but that's not really a usable figure because the number would vary greatly depending on your driving and charging habits.

Q: So how do I know what would the car's real fuel economy be?

A: A table in the lower right of the fuel economy sticker (under "Charging Routines") gives a better idea of what the Volt's actual gas mileage might be depending on how far you drive it before recharging the batteries. These figures are calculated solely on the basis of the distance traveled after a full charge and the amount of gasoline used.

If driven less than the estimated 35-mile range the Volt can run on a single charge, an actual Miles Per Gallon figure isn't applicable because you'd be using no gasoline at all. (There have been real-world reports of Volts going much farther than 35 miles on a charge.) If driven 45 miles before recharging, with just 10 miles of that using gas, the Volt would get 168 miles per gallon, according to the EPA fuel economy label. If driven 75 miles, it would get an estimated 69 mpg.

Q: All I really care about is saving money on gas. How much will an electric car save me?

A: For a purely electric car like the Nissan Leaf, a cost comparison would be very straightforward. The EPA assumes you drive 15,000 miles a year, gas is at $3.20 a gallon, and electricity costs 11 cents per kilowatt hour. In those conditions, it's estimated the Leaf would cost you $561 a year in electricity. For the Nissan Versa, a similar-sized gas-powered car, it'd be about $1,444 a year for gasoline. So your fuel savings compared to the Leaf would run $883 a year.

Things are more complicated for the Chevrolet Volt since how much it costs to drive will depend largely on your driving and charging habits. Even if it's charged every single day and never uses a drop of gasoline, the heavier Volt will cost about $601 a year in electricity -- $40 more than the Leaf. To take matters to the other extreme, if you never charged it at all and ran it only on gas, the Volt would cost $1,302 in gasoline. (Chevrolet's Cruze, which is very similar to the Volt but runs on gas, will cost about $1,600 a year in gasoline by comparison, according to the EPA. So the Volt still wins there.)

To come up with a more-or-less realistic annual fuel cost, you'll need to consider your own long-term driving routine. For instance, say you drive less than 35 miles on most days, have occasional days in which you cover more ground and, once in a while, take much longer trips. Figure you'll drive about a third of the time using gas, and the rest of the time on a charge. That means you'll spend about $400 a year on electricity, and another $533 on gas for a total annual fuel cost of roughly -- OK, very roughly -- $933. That's an annual savings of about $667 compared to a Chevy Cruze.

Q: So does an electric car make sense for me?

A: It depends largely on what "makes sense" means for you. It also depends on a lot of other factors.

If you simply want to save money, an electric car probably wouldn't make sense for you right now. You'd save money on gasoline but pay more up front for the car. Even that's not so simple, though. We don't know yet how well these cars will retain their resale value and how much insurance companies will charge to cover them. An electric car could end up costing you less to drive if it holds its value really well and is much cheaper to insure.

If you want to do your part to reduce the impact of burning fossil fuels, out of concern for the environment or national security, an electric car may make sense. There are arguments about whether it's better for the environment to burn gasoline or use coal-fired electricity but if the electricity comes from clean or renewable sources, the electric car is the clear winner.

And, at least in the case of the Chevrolet Volt and Nissan Leaf, you'll be getting a good car for your money.