How Green Is a Tesla, Really?

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Sept. 9 2013 5:15 AM

How Green Is a Tesla, Really?

As green as a golf cart? A Prius? An SUV?

A view of a fully electric Tesla car on an assembly line at the new Tesla Motors car factory in Tilburg, the Netherlands, during the opening and launch of the new factory, on August 22, 2013.
A Tesla assembly line. Just because the company's cars are electric-powered, doesn't mean they have no environmental impact.

Photo by Guus Schoonewille/AFP/Getty Images

The knock on electric cars has always been the same: They’re great for the environment, but they’re pokey and impractical, and nobody wants to buy one. The stunning success story of the Tesla Model S has, improbably, flipped that equation. It’s blazingly fast, surprisingly practical, and everyone wants to buy one. But now some critics are asking: How green is it, really?

Will Oremus Will Oremus

Will Oremus is Slate's senior technology writer.

The quick answer: If current trends hold, it could be pretty darn green in the long run. But as of today, the calculation isn’t as straightforward as you might think. Depending on whom you ask, what assumptions you make, and how you quantify environmental impact, the answer could range from “greener than a Prius” to “as dirty as an SUV.” And where the Tesla falls on that spectrum depends to a surprising extent on where you live and how much you drive it.

Electric cars are squeaky clean, of course, in the sense that they don’t burn gas. With no engine, no gas tank, and no exhaust, they’re considered to be zero-emissions vehicles. But there’s more to a vehicle’s environmental impact than what comes out of the tailpipe. The Tesla doesn’t run on air. It runs on electricity, which in turn is generated from a range of different sources, from nuclear fission to natural gas to the darkest, dirtiest fossil fuel of them all: coal.

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So if you’re going to stack a Tesla’s per-mile emissions against those of a gas-powered vehicle, you’ll need to start by looking at the composition of the electrical grid. Nationally, the grid is roughly 40 percent coal, 25 percent natural gas, 20 percent nuclear power, and about 10 percent renewable sources, led by hydroelectricity. So it’s fair to say that your average Tesla is powered in large part by burning fossil fuels.

Tesla acknowledges this, and insists that its cars are still far cleaner than their internal-combustion competitors. That’s because battery-powered cars are more efficient at converting their stored energy into forward progress. A Model S can travel upwards of 265 miles on a single charge of its 85 kilowatt-hour battery, which equates to less than 3 gallons of gas. Its official EPA miles-per-gallon equivalent is 89, far greater than a standard Toyota Prius.

For any given Model S, though, the emissions-per-mile depend heavily on the mix of energy sources that go into your local grid. According to Tesla’s own emissions calculator, if you’re driving your Model S in West Virginia—where the power mix is 96 percent coal—you’re spewing some 27 pounds of CO2 in a typical 40-mile day, which is comparable to the amount you’d emit in a conventional Honda Accord. Indiana, Kentucky, and Ohio aren’t much better. On the other hand, if you’re charging your Tesla in California, where natural gas supplies more than half the electricity—or, better yet, Idaho or Washington, where hydroelectricity reigns—your per-mile emissions are a fraction of that amount. Congratulations: Your Model S is a clean machine after all.

Or is it? In May, a market analyst named Nathan Weiss prompted spit-takes throughout the clean-energy world in May with an incendiary post on the financial news site Seeking Alpha. The headline: “Is the Tesla Model S Green?” Weiss’ answer: a resounding, math-heavy, 6,500-word “No.” In fact, Weiss argued, the Model S is in many ways dirtier than a Jeep Grand Cherokee—and nearly as dirty as a Ford Expedition, one of the largest SUVs on the market. That’s an extreme position, and large swaths of Weiss’ argument were readily rebutted by electric-car advocates. Facing a barrage of criticism, Weiss soon revised his calculations, but still insisted that the Model S’s effective CO2 emissions exceeded those of a smaller SUV like the Toyota Highlander.

Weiss’ post was followed in late June by a report in IEEE Spectrum from Ozzie Zehner, a one-time electric-car enthusiast turned outspoken critic. His report, titled “Unclean at Any Speed,” cites studies that find electric cars are no cleaner—and in some cases less clean—than gas-powered cars.

How could that be? Critics point to three main factors that make a Tesla dirtier than the EPA’s ratings, or the company’s own data, would suggest. First, coal-burning power plants emit not just CO2 but also other noxious gases like nitrogen oxides and sulfur dioxide—and in far greater quantities than gas-powered cars. “If a smog-testing center could measure the effective emissions of a Tesla Model S through a tailpipe,” Weiss wrote, “the owner would face fines, penalties, or the sale of the vehicle under state ‘clunker buyback’ programs.” (This problem isn’t Tesla’s fault, obviously, and it would vanish with a cleaner energy grid.)

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