Tesla Motors has an army of loyalists who either swear by the company’s sleek and stealthy high-end luxury electric cars or dream of getting their hands on one. They share company co-founder Elon Musk’s vision of ending the 130-year reign of the internal combustion engine by steering drivers away from gasoline and toward electricity.

But hold on to your smugness, Tesla owners. Not all electric cars are the same, and until the U.S. more fully embraces renewable energy sources, buying an electric car isn’t necessarily the greenest option out there. In fact, some hybrid vehicles can be greener options than fully electric cars, according to people who study the sources of carbon emissions that cause global warming. (Sorry climate deniers, you’ve lost that debate.)

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Important factors in determining carbon emissions include the weight of the vehicle, driving habits, and the source of the electricity that charges your car. Likewise, it can be a much greener choice to keep the perfectly functional car you have, rather than go out and buy a new one.

“If you are a relatively low-mileage person, you should stick with your gas-powered car,” Mike Berners-Lee, a leading expert in measuring the amount of greenhouse gases released by the products we buy, told Salon. “When the time comes to buy a new car, you should buy a nice, small electric car, and you should still keep the mileage down. You should still try to find other forms of transport when you can, and you should share transportation as much as you can.”

One of the reasons why buying a new car is a problem is the vehicle’s so-called embodied carbon, meaning all of the energy that was used to build the car from scratch — including the extraction and processing of raw materials, and shipping parts and vehicles across oceans in filthy bunker-fuel burning cargo ships. Every time you roll off the dealer’s lot in a new set of wheels — electrified or not — your personal carbon footprint grows immensely.

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Berners-Lee, the British author of “How Bad are Bananas?: The Carbon Footprint of Everything,” calculated that making one midsize sedan like the Ford Mondeo (known in the U.S. as the Ford Fusion) generates about 17 metric tons of carbon dioxide; three year’s worth of gas and electricity consumed by a typical British household produces about the same amount. In general, the amount of carbon it takes to manufacture a car scales depending on its mass: an SUV like the Land Rover Discovery creates about twice the carbon yield of the Ford Mondeo merely in production, where as a compact car only yields about 6 metric tons of carbon.

Electric cars aren’t much different than gas-burning vehicles in this regard. In fact, the U.S. Union of Concerned Scientists estimates that it takes about 15 percent more embodied carbon to produce an electric vehicle (EV) than it does to manufacture a gasoline-powered car, largely because of the materials and fabrication processes used to make the battery packs.

“For a full-size [electric] car, it can be a higher percentage increase in the emissions” of embodied carbon, David Reichmuth, senior engineer in the Clean Vehicles Program at the Union of Concerned Scientists, told Salon.

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However, Reichmuth is quick to point out that while an electric car is modestly more polluting to manufacture, it more than makes up for the difference over the life of the vehicle. A study Reichmuth co-authored and released in 2015 shows that by the time a mid-size electric car hits 135,000 miles, it will have produced half of the emissions of a comparable gasoline-powered sedan.

Electric car emissions are measured using the carbon output of electrical power plants. Since most electric power is generated from fossil fuels, electric cars contribute to greenhouse gas emissions unless all of the electricity they consume comes from renewable sources like wind and solar.

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“The principles need to be that we make the cars as efficient as we can, and part of that is moving to electric,” Reichmuth said. “We need to make the fuels as clean as we can and that means shifting oil to electricity, and electricity that’s made without emissions.”

So, if you own an electric car and you keep it for years, then you’re doing the best you can, right?

Well, not necessarily, says John Scofield, a professor of physics at Oberlin College. “The more weight you add to a car, the more rolling resistance,” Scofield told Salon.

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There’s an immense difference in efficiency between a 5,400-pound electric SUV like the Tesla Model X and a plug-in gas-electric hybrid like the Chevrolet Volt, which weighs nearly 2,000 pounds less. While the Volt can travel only about 50 miles on battery power alone before the gasoline engine kicks in, it utilizes energy more efficiently because it has less weight to pull around.

Scofield, who in 2012 testified to a House subcommittee meeting on energy efficiency, also pointed out that the more people drive electric cars, the less renewable energy is available. Because of the way utilities trade power across state lines, even if you live in a city like Seattle that receives most of its electricity from renewables, you might still be sometimes tapping power provided from fossil fuels.

“Nobody in Seattle is making more hydroelectric power,” he said. “If you didn’t use the electricity [from a renewable source] it was going somewhere else.” But as demand for electricity increases beyond what can be provided by a renewable energy source at any given time, the grid has to be supported with power from non-sustainable sources. “The replacement energy will come from burning gas. People don’t get that,” Scofield added.

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There are numerous reasons why a plug-in hybrid may actually be greener than an electric car. For those supremely interested in owning an eco-friendly vehicle, the most important factor is weight; the heavier and bigger the car, the more energy you’re wasting. Another major factor is whether you need to buy a new car at all. Because of the embodied carbon issue, it is better to get a few more years of life out of your gas-powered car, rather than buy a new electric car, according to Berners-Lee.

“We need to make the most of our embodied carbon,” he said. “It’s no good being fashion freaks with our cars. It just doesn’t work environmentally. You have to buy a lightweight quality car, take care of it and keep it on the road.”

And of course, drive it as little as possible when other options are available. Or better yet, embrace public transit or car sharing and help save the world by reducing the number of cars in it — electric or otherwise.