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Electric-Car Efficiency: Forget MPGe, It Should Be Miles/kWh

Two days ago, we ran down all the rated ranges of every plug-in electric car offered for sale in the U.S. market this year.

We did not include ratings for efficiency.

In part, that’s because the unit the EPA uses doesn’t actually have much to do with the electricity owners use to recharge the battery packs that power their electric cars.

Instead, the EPA has chosen to use an artificially created metric called Miles Per Gallon Equivalent, or MPGe.

Setting aside the issue that when a plug-in car is traveling on battery power, there are no gallons of gasoline involved anywhere in the process, here’s how it works.

The efficiency rating measures how far an electric car can travel on the amount of electricity contained in 1 gallon of gasoline (which, for the record, it says is 33.7 kilowatt-hours).

Note that the gallon of gasoline costs $3.50 to $4.75–while the equivalent energy content delivered over the electric grid to your car costs from $1 to $8, depending on your local electricity rates.

But it’s impossible to buy a MPGe, and so the efficiency measure is useless for actually calculating the cost of operating an electric car.

Instead, electric-car drivers buy kilowatt-hours of electricity, most of which goes to power their cars (plus some for overhead like keeping the pack at the optimal temperature, in many cars).

Of course, the non-linear Miles-Per-Gallon measure isn’t very helpful in calculating how much it’ll cost to go a given distance.


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