We are so used to seeing “Miles Per Gallon” estimates rattled off during car commercials, but how do we represent the efficiency of an alternative-fuel vehicle? The EPA’s answer is the “Miles Per Gallon Equivalent,” or MPGe.

For electric cars, models display kilowatt-hours per 100 miles in addition to the MPGe value. This represents how much electricity is required to power the respective car for 100 miles. This is a measure of energy consumption.

The MPGe, on the other hand, is a measure of energy efficiency. A vehicle with a higher MPGe uses less electricity go a longer distance, i.e. it is more efficient. But what is the value of one MPGe? Technically it is the energy contained in one gallon of gas converted to kilowatt-hours. If a car uses 33.7 kWh of electricity, it has used up just as much energy as a gallon of gas contains. So if if a car takes in exactly 33.7 kWh and drives exactly 100 miles on that charge, it has an MPGe of 100.

Still a little confusing, right? It’s not the greatest metric, but it does point out just how efficient electric cars are compared to conventional ones. Maybe you can’t decide which EV to buy from the MPGe, but it sure leaves your gas car’s MPG in the dust.