Kilowatt-Hours to MPG-Equivalent
So how does the EPA get from 32 kWh/100m to 105 MPGe? To create the mpg equivalent, the EPA uses an established energy standard of 115,000 BTUs (British thermal units) per gallon of gasoline. Grossly oversimplified, this means that if you ignited 1 U.S. gallon of unleaded gasoline, it would generate that much heat. To create the same amount of heat, you would need 33.7 kilowatt-hours of electricity.
So if the 2014 Ford Focus EV could travel 100 miles on 33.7 kWh of electricity (the energy equivalent of 1 gallon of gasoline), it would receive an mpg equivalency of 100 MPGe. But the Focus EV actually requires slightly less than the 33.7 kWh to travel 100 miles (in this case, 32 kWh), so it received an mpg equivalency rating of 105 MPGe.
The EPA label provides both city and highway numbers as well as the overall average fuel economy. For EVs, which tend to do better in stop-and-go driving, where regenerative braking can help contribute to the battery's charge, city fuel efficiency is typically better than highway. That's the reverse of most gasoline vehicles. The Focus EV receives a 110 MPGe rating for city driving but scores only 99 MPGe on the highway.
What does all this have to do with fuel economy? Not much.
The MPGe rating is really only useful for comparing the relative energy consumption of gasoline (or hybrid) cars with that of electric cars. The Focus EV uses the energy equivalent of 1 gallon of gasoline for each 105 miles of travel, compared to a hybrid Prius, which would use roughly 2 gallons of gasoline for every 100 miles it travels. Far more pertinent for electric car owners focused on cost is the kilowatt-hours-per-100-miles rating (kWh/100m), which shows you how efficient the vehicle is at converting electricity into miles traveled. The kWh/100m rating is the new EV mpg, and from a pocketbook standpoint, lower is better.