Say you're a software developer holding down a six-figure salary at Microsoft in Redmond. You read the news; maybe you even heard World Meteorological Organization secretary-general Michel Jarraud on TV saying "we're running out of time" to deal with global warming, ahead of next week's climate summit in New York. You want to do something personally to lower your carbon emissions. Why not buy an electric car? Now is an especially good time—it's National Drive Electric Week!

So you head to the Bellevue Tesla dealership and drop 70 grand on an all-electric Model S. After all, the highly acclaimed, top-selling sedan has no exhaust pipe, uses no gasoline, and charges in your garage.

But if you charge up your Tesla on the Eastside, a fifth of the electricity coming through Puget Sound Energy's grid into your car originates at a dirty coal plant in Colstrip, Montana—the eighth-largest source of greenhouse gases in the entire country. This erases much, though not all, of the car's environmental value compared with a conventional gasoline-powered car. Using PSE's 2012 emissions figures, an analysis by the Sightline Institute's Clark Williams-Derry shows that a Tesla Model S with an 85 kilowatt battery emits 0.50 pounds of "carbon dioxide equivalents"—a standard unit of measure for greenhouse gas emissions—per mile, compared with 0.51 pounds per mile for a Toyota Prius C. (This assumes a 5 percent rate of energy loss between the power plant and your outlet; PSE did not provide its specific loss rate, but 5 percent is pretty average.)

Bottom line? "If you're drawing power from the mix of generators that PSE owns, a Tesla comes out neck and neck with a plain-vanilla Prius," Williams-Derry says. "Having a lot of coal in your generation mix significantly reduces the benefits of driving a Tesla."

Hop across the water to Seattle to charge it, and your electric car becomes magically better for the earth. Seattle City Light's grid gets less than 1 percent of its energy from coal—bringing your Tesla's carbon emissions down to near zero. That disparity has driven some electric-car owners to "try to get a preferential charge in Seattle," says Seattle Electric Vehicle Association legislative issues coordinator John McCoy—rather than on the Eastside. "We want our members to plug into the cleanest grid that they can," he says.

So just how dirty is PSE's grid? Washington's oldest utility, which serves more than a million customers around the Puget Sound region, gets 30 percent of its power from fossil fuels, with two-thirds of that coming from the Colstrip Generating Station in Montana.

For the people of Colstrip, the power plant has been a scourge. Its coal-ash ponds have been leaking toxic chemicals into the surrounding groundwater and storm water for decades. In 2004, the plant owners—including PSE—paid out millions of dollars in a settlement with locals who sued over being unable to use their own water. "If Colstrip were out in Puget Sound... it would have been closed down a long time ago," says Anne Hedges, deputy director of the Montana Environmental Information Center. "But in this case, it's a thousand miles out of the way. Out of sight and out of mind."

"It's dirty, it's old, it's inefficient," she adds. "It's going to be a nightmare to clean up."

But the tide is beginning to turn. In February, the Washington Utilities and Transportation Commission put PSE on notice. After reviewing the utility's 20-year plans, commissioners said they were "unable to conclude that the utility's continued reliance on older coal-fired power plants in Colstrip, Montana, is justified." The commission questioned the prudence of making any new investments in the plant versus "a closure or partial-closure plan." Its findings came after an unusual deluge of public comments, many of them arguing against PSE's reliance on the coal plant.

"Doing... the right thing for our planet is what PSE is all about," the utility's website says. I asked PSE spokesman Grant Ringel whether the utility has a plan to transition away from the Colstrip plant. He said, "Colstrip is a very important part of today's Northwest energy infrastructure because it's such a big plant and it produces energy at such a low cost." They have to balance financial costs with environmental concerns, he said.

Pressed repeatedly on PSE's future plans, he said that while there's "no specific decision... about changing the course at Colstrip, that doesn't mean it won't happen."

Puget Sound Energy actually has the largest ownership stake, out of a consortium of utilities, of the Colstrip coal plant. And in April, Governor Jay Inslee issued an executive order that makes shifting Washington's energy away from coal sources a top priority. "Puget Sound Energy is perfectly positioned," says Hedges. "They can transition off Colstrip. They just have to make the decision to do so... There is no better entity to be a leader on this."

With National Drive Electric Week, a major international climate summit coming up, and record levels of greenhouse gases in the atmosphere, there's no time like the present. recommended