Naysayers love to point out how dirty the electricity grid mix is when it comes to charging electric vehicles. Curmudgeons are eager to jump into any conversation about EVs to enlighten the lucky listeners about how plug-in cars contribute to pollution, sometimes even throwing in a dash of climate-change denial for good measure. (Thanks, buddy. Pray, tell me more about the plight of oppressed SUV owners.)

Unless someone buys an EV just because they think they're cool (which, yeah, they often are), they probably have at least a passable understanding of their environmental pros and cons. As many EV owners are already aware, location has a lot to do with any particular plug-in car's carbon footprint. Still, there's always more to know, and knowledge is not a bad thing, especially if one uses it to do the right thing. That's why this handy-dandy map from Carnegie Mellon University is so interesting.

CMU researchers have compiled information about the lifecycle greenhouse gas emissions of various EVs based on where they're charged, as compared to gasoline-powered vehicles. The researchers looked at the Nissan Leaf, Chevrolet Volt, and Prius Plug-In Hybrid versus the gasoline-dependent Toyota Prius hybrid and the stop-start-equipped Mazda3 with i-ELOOP and compared grams of CO2 emitted per mile.

CMU takes into account the grid mix, ambient temperature, and driving patterns.

CMU takes into account the grid mix based on county, as well as ambient temperature and driving patterns in terms of miles traveled on the highway or in the city. For instance, if you drive a Nissan Leaf in urban areas of California, Texas, or Florida, your carbon footprint is lower than it would be if you were driving a standard Toyota Prius. However, if you charge your Leaf in the Midwest or the South, for the most part, you've got a larger carbon footprint than the Prius. If you live in the rural Midwest, you'd probably even be better off driving a Mazda3.

Throughout the country, the Chevrolet Volt has a larger carbon footprint than the Toyota Prius, but a smaller one than the Mazda3 in a lot of urban counties in the US. The Prius and Prius Plug-In are relatively equal across the US.

Having trouble keeping it straight? That's not surprising. The comparisons between plug-in and gasoline vehicles are much more nuanced than the loudest voices usually let on. That's why the graphic is helpful (at least for the time being – the grid continues to get cleaner as more renewable energy comes online). If you want to get into the nitty-gritty details of the study, though, it's openly available online. You can also read more in the press release below.

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Want An Electric Vehicle? The One You Should Buy Depends on Where You Live

PITTSBURGH, April 26, 2016 /PRNewswire/ -- From the Tesla Model 3 to the Nissan Leaf, electric vehicles are all the rage these days—but before consumers head to the dealership, they should take a moment to consider a few factors. Their city's climates, how much highway driving they will be doing, and their regional electricity grids all contribute to their vehicles' carbon footprints. In other words, if they buy the wrong car for where they live, their good intentions may leave an even bigger carbon footprint than a conventional gasoline-fueled vehicle.

Researchers from Carnegie Mellon University have published the first study that accounts for the combined regional influence of electricity grid emissions; driving patterns, such as distance and highway vs. city driving; and temperature on the life cycle emissions of five different types of vehicles. The vehicles compared were the Nissan Leaf, the Chevrolet Volt, the Toyota Prius hybrid and plug-in hybrid models, and the Mazda 3.

"Most electric vehicle buyers assume that they're helping the environment by purchasing an electric or hybrid vehicle, but there's more to it than simply making the purchase," said study co-author Jeremy Michalek, a Carnegie Mellon University professor of mechanical engineering and of engineering and public policy. "Where a consumer lives plays a larger role than one might think in determining which vehicle will have the lowest greenhouse gas emissions."

When the vehicles' emissions are compared, it turns out that the entirely battery-run 2013 Nissan Leaf produces lower greenhouse gas emissions than the entirely gasoline-run 2013 Toyota Prius in urban areas of Texas, Florida, and much of the southwestern United States—but in the rest of the country, most notably in the Midwest and the South, the Leaf emits more greenhouse gases than the Prius.

Current federal policy for plug-in electric vehicles is fairly uniform across the United States, although different states have different policies to encourage electric vehicle sales, including subsidies and mandates. The study identifies regions where electric vehicles offer the largest reductions of carbon dioxide emission to help inform future regional policy decisions.

"Electric vehicles offer the largest benefits for urban drivers in mild-climate regions with a clean electricity grid, such as San Francisco or Los Angeles," says Michalek. "In the rural regions of the colder, coal-heavy Midwest, electric vehicles are often higher emitting than comparable gasoline vehicles today."

So, if consumers are in the market for a new car, they should make sure to take their locations—and their good intentions—into account.

Check out which electric vehicles consumers should buy based on where they live: engineering.cmu.edu/emissions-maps

More comparisons between vehicle emissions can be found at Environmental Research Letters in the full article, titled "Effect of Regional Grid Mix, Driving Patterns and Climate on the Comparative Carbon Footprint of Gasoline and Plug-in Electric Vehicles in the United States." The researchers involved in this study were Jeremy Michalek; Tugce Yuksel, a mechanical engineering Ph.D. student; Inês Azevedo, an associate professor of engineering and public policy; Chris Hendrickson, professor of engineering and public policy and of civil and environmental engineering; and Mili-Ann Tamayao, who began work on the project while an engineering and public policy Ph.D. student at Carnegie Mellon University. Tamayao continued contributing to the paper after she graduated and joined the University of the Philippines.


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