Decoding MPG-e

Mpg-e was actually pretty tricky to develop. Gasoline is measured in gallons, which is a measure of volume. If you were to measure electricity use of EVs and plug-in hybrids, you’d use kilowatt-hours – which is the equivalent power output of 1,000 watts over one hour. You’re probably familiar with kilowatt-hours because that’s the unit electric companies use to compute your electric bill. So, when coming up with mpg-e, the EPA had to figure out how to convert a measure of energy use over time into a measure of volume. This isn’t just figuring out a way to compare apples to oranges. It’s more akin to figuring out a way to compare apples to land speed records.

Even though a gallon is a measure of volume, the gasoline in that gallon contains energy. You release that energy by burning it. When you burn a gallon of gas (which isn’t something we recommend you do in your backyard just because you’re bored), it generates the same amount of heat as about 34 kilowatt-hours of electricity (the EPA actually uses 33.7, but we’re rounding up). So, 34 kilowatt-hours is the rough equivalent to one gallon of gas that the EPA uses to calculate mpg-e.

Now, for most electric car shoppers, the kilowatt-hours of electricity needed to drive 100 miles is going to be the key number, because that will determine how much you pay for electricity when charging your car. However, with mpg-e, you can translate that number into numbers that owners of gasoline-powered cars will understand. For comparison’s sake, the gas-powered Fiat 500 with an automatic transmission gets a combined fuel economy rating of 30 mpg. Based on mpg-e, the 500e uses 74 percent less energy to travel the same distance as the gas-only 500.

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