In the article “Plug-In Vehicles” (HP151), Andy Kerr points out that 1 gallon of gas has the “energy equivalent of 33.7 kWh” and that an electric vehicle that gets 3 miles per kWh would be equivalent to 100 miles per gallon [of gasoline].
This is a bit misleading because not all the energy in 1 gallon of gas can be turned into electricity. Even with the best engine technology, the second law of thermodynamics puts a limit on the percent of heat energy that can be converted.
Typically, utilities turn about one-third of the heat generated into electricity. The best they can do is close to 50%. This puts the equivalent energy consumption of 3 miles per kWh between 33 miles per gallon and 51 miles per gallon.
Larry Schlussler • Arcata, California
The U.S. EPA developed “miles per gallon-equivalent” (MPG-e) because consumers are used to mpg for their gasoline- (or even diesel-) powered cars and needed a way to compare. The EPA says “Electric vehicles convert about 59% to 62% of the electrical energy from the grid to power at the wheels—conventional gasoline vehicles only convert about 17% to 21% of the energy stored in gasoline to power at the wheels.” (See bit.ly/EPA_EV.)
The methodology they developed (described here: bit.ly/MPGequivalent) came up with 1 gallon of gasoline equaling 33.7 kilowatt-hours. In other words (and yes, I would agree it is approximate, if not misleading), a car can travel the same distance on 1 gallon of gas or 33.7 kWh of electricity.
[Editor’s note: If you scroll to the bottom of the Wiki page that Andy mentions above, it details “well-to-wheel” comparisons, “accounting for the upstream efficiency of electricity generation and transmission, etc. In the example provided by the U.S. DOE in its final rule, an electric car with an energy consumption of 265 Wh per mile in urban driving and 220 Wh per mile in highway driving resulted in a petroleum-equivalent fuel economy of 335.24 miles per gallon.”]