Kilowatt-hours per 100 Miles (kWh/100m): Why Less is Better

BY Ben Bowlin / POSTED June 15, 2012
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The all-electric Nissan Leaf is displayed during the opening ceremony of the company’s new headquarters in Yokohama, Japan, on Sunday, Aug. 2, 2009. (AP Photo/Itsuo Inouye)

Over the past week, Scott and I have taken a closer look at the new EPA car labels, which include some interesting (and, perhaps, confusing) metrics. If you check out the sample electric car label on the EPA’s site, you’ll notice a few new additions. The biggest number is the MPGe rating, and there are three other, smaller numbers listed on the right. The first two make sense — they’re MPGe ratings for city and highway driving. But that third number can throw some folks for a loop: What the heck is this ‘kilowatt-hours per 100 miles’ business? Are you telling me the number’s supposed to be that low?

Well, yes, I am. Ideally the kWh/100m measurement should be as low as possible. It’s a bit of a switch-up, and the best way to explain it is through comparison with MPG. Both calculations measure the ratio of energy used and distance traveled. With an MPG rating, we’re looking at the distance (miles) first, and the energy (gasoline) second. We want the highest possible MPG rating, because that number describes the distance possible on one gallon of gas. If we were to reverse the order and measure something like gallons of gas per mile (GPM), then we’d have a radically different number.

And that’s how the kWh/100m metric works. We’re listing the energy consumed first, and the distance traveled second. With this in mind, it’s easy to see why we want those kWh/100m numbers to be as low as possible. A lower number here equals a smaller amount of energy used.

What’s the moral of the story, you might ask. It’s simple: When it comes to kWh/100m, lower is better. So don’t be fooled. If you see a rating of something like ’195kWh/100m,’ run for the hills. That’s either a typo or the world’s worst electric car.

And one last thing: If you think that rating vehicles in terms of GPM is a better system than the good ol’ MPG, you might be surprised to learn that you’re not alone. Loads of folks think the GPM is a more intuitive way to display this ratio.

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