Measuring Efficiency in a Battery-Powered Electric Car
The U.S. Environmental Protection Agency this week issued a fuel economy rating of 119 MPGe to the Chevrolet Spark EV, making it the efficient car in the United States. The all-electric subcompact goes on sale this summer in California and Oregon. The news begs this question: What is that little "e" doing at the end of MPGe?
First, the context. For the better part of a century, consumers have used the well-known Miles Per Gallon (MPG) number to describe a vehicle’s efficiency. Car owners have, in fact, grown so familiar with using MPG (and what it means) that it’s now being applied to electric vehicles that don’t have gas tanks with gallons of liquid fuel—but instead to cars that use battery packs with kilowatt-hours of electric energy.
Consumers who bother to read their electric bill will recognize kilowatt-hours. But even if you take the time to study your utility bill—who bothers to do that?—drivers have trouble wrapping their minds around kilowatt-hours of energy related to driving, according to focus groups conducted by the E.P.A.
Participants in the E.P.A. study instead said they want to use some kind of equivalent to the familiar MPG metric. And thus MPGe (miles per gallon plus "e" for equivalent) has been applied to plug-in car window stickers since November 2010 when the Nissan LEAF all-electric hatch hit the market.
Based on the idea that one gallon of gasoline is the equivalent of 33.7 kilowatt-hours of electricity, here are efficiency ratings for the leading electricity-powered cars:
- Chevrolet Spark EV – 119 MPGe
- Honda Fit EV - 118 MPGe
- Ford Focus Electric - 105 MPGe
- Nissan LEAF - 99 MPGe
As you can see, the relative efficiency of the most electron-hungry EV compared to the stingiest one is fairly minor. An EV rated at 120 MPGe will go 25 percent further on a kilowatt-hour compared to a 90 MPGe EV. But that difference mostly has to do with the weight of the vehicle (and with its aerodynamic qualities) rather than anything else that engineers have done.
And as with gas cars, the single most important factor related to efficiency is the driver. If you drive a small EV efficiently, you can get four miles or better on a kilowatt-hour of juice. Drive a bigger EV for rip-roaring fun, and expect three miles or less on a kWh.
At the end of the day, the 100 MPGe-plus numbers are good for EV bragging rights, but we shouldn’t over-think electric car efficiency. All these numbers really mean to most people is: Wow, that’s a high number. Electric cars are two or three times as efficient as most gas cars.