Ever wondered just how efficient those electric and hybrid cars really are? Well the EPA has too, and to answer that question they’ve created something called “miles per gallon gasoline equivalent” or MPGe. It’s a number included on the window sticker of every new electric and plug-in hybrid vehicle, but for those who aren’t automotive engineers its meaning isn’t as obvious. Read on to learn more from DCH Toyota of Oxnard.
Measuring efficiency
Miles per gallon, or MPG, is a familiar measure of fuel efficiency: it shows how far a vehicle will travel on a gallon of gasoline. MPGe attempts to do the same for alternative energy sources. As the EPA puts it, “A gallon of gasoline-equivalent means the number of kilowatt-hours of electricity, cubic feet of CNG, or kilograms of hydrogen that is equal to the energy in a gallon of gasoline.”
So how much energy is there in a gallon of gasoline?
When a gallon of gasoline is burned it releases the equivalent of 33.7kWh of electricity. To put it in perspective, one kiloWatthour is like leaving ten 100W lightbulbs on for sixty minutes, so 33.7 kWh is 337 100watt bulbs burning for an hour.
So MPGe is how far the car would travel on 33.7kWh of electricity. Interestingly, it tends to be quite a long way: many electric vehicles will get 90 to 100MPGe. That’s because electric motors are inherently more efficient than gasoline engines with all their internal friction.
Running costs
Unfortunately, MPGe doesn’t say how much an electric car will cost to run per mile, any more than MPG does for a gasoline fueled car. The missing ingredient in both cases is the cost of the fuel.
Although it fluctuates, gasoline has been around $3.50 for some time. That means a compact car that turns in 35 MPG costs 10 cents per mile to run. The price of electricity varies too, but 10 cents per kilowatt hour isn’t too far from what most people pay, and it makes the arithmetic easier too! So 33.7kWh costs around $3.37. So an electric car rated at 95MPGe will travel 95 miles on $3.37 of electricity. That’s 3.5 cents per mile, so it does save money compared to gasoline.
But that’s not the whole story.
Flawed but still useful
Even though the sticker on the gas-powered car says 28/35 City/Highway MPG, few people see those numbers in real-world driving. That’s because the EPA measures gas mileage on a strictly defined set of tests that don’t reflect most people’s commute.
What makes it useful though is that all cars are tested the same way. That means it’s possible to say that car A uses less gas than car B.
MPGe extends this comparison to cars that run on other fuels, like electricity. As a result, it’s possible to compare the efficiency of gasoline, electric, and even hydrogen-powered vehicles.
