Around the world, governments and automakers are promoting electric vehicles as a key technology to curb oil use and fight climate change. General Motors has said it aims to stop selling new gasoline-powered cars and light trucks by 2035 and will pivot to battery-powered models. This week, Volvo said it would move even faster and introduce an all-electric line-up by 2030. But as electric cars and trucks go mainstream, they have faced a persistent question: Are they really as green as advertised?

While experts broadly agree that plug-in vehicles are a more climate-friendly option than traditional vehicles, they can still have their own environmental impacts, depending on how they’re charged up and manufactured. Here’s a guide to some of the biggest worries — and how they might be addressed. Broadly speaking, most electric cars sold today tend to produce significantly fewer planet-warming emissions than most cars fuelled with gasoline. But a lot depends on how much coal is being burned to charge up those plug-in vehicles. And electric grids still need to get much, much cleaner before electric vehicles are truly emissions free.

One way to compare the climate impacts of different vehicle models is with this interactive online tool by researchers at the Massachusetts Institute of Technology, who tried to incorporate all the relevant factors: what it takes to manufacture the cars, how much gasoline conventional cars burn and where the electricity to charge electric vehicles comes from. If you assume electric vehicles are drawing their power from the average grid in the United States, which typically includes a mix of fossil fuel and renewable power plants, then they’re almost always much greener than conventional cars. Even though electric vehicles are more emissions-intensive to make because of their batteries, their electric motors are more efficient than traditional internal combustion engines that burn fossil fuels.

An all-electric Chevrolet Bolt, for instance, can be expected to produce 189 grams of carbon dioxide for every mile driven over its lifetime, on average. By contrast, a new gasoline-fueled Toyota Camry is estimated to produce 385 grams of carbon dioxide per mile. A new Ford F-150 pickup truck, which is even less fuel-efficient, produces 636 grams of carbon dioxide per mile.

But that’s just an average. On the other hand, if the Bolt is charged up on a coal-heavy grid, such as those currently found in the Midwest, it can actually be a bit worse for the climate than a modern hybrid car like the Toyota Prius, which runs on gasoline but uses a battery to bolster its mileage. (The coal-powered Bolt would still beat the Camry and the F-150, however.) “Coal tends to be the critical factor,” said Jeremy Michalek, a professor of engineering at Carnegie Mellon University. “If you’ve got electric cars in Pittsburgh that are being plugged in at night and leading nearby coal plants to burn more coal to charge them, then the climate benefits won’t be as great, and you can even get more air pollution.”

The good news for electric vehicles is that most countries are now pushing to clean up their electric grids. In the United States, utilities have retired hundreds of coal plants over the last decade and shifted to a mix of lower-emissions natural gas, wind and solar power. As a result, researchers have found, electric vehicles have generally gotten cleaner, too. And they are likely to get cleaner still.