With such heavy batteries, an electric car’s carbon footprint can grow quite large even beyond the showroom, depending on how it’s charged. Driving in France, which relies heavily on nuclear power, will spit out a lot less CO2 than Germany, where 40 percent of the grid burns on coal.

“It’s not a great change to move from diesel to German coal power,” said NorthVolt AB CEO Peter Carlsson, a former Tesla manager who is trying to build a 4-billion-euro ($4.6 billion) battery plant in Sweden that would run on hydropower. “Electric cars will be better in every way, but of course, when batteries are made in a coal-based electricity system it will take longer” to surpass diesel engines, he said.

To be sure, other studies show that even in coal-dominant Poland, using an electric car would emit 25 percent less carbon dioxide than a diesel car, according to Transport & Environment Brussels, a body that lobbies the European Union for sustainable environmental policy.

The benefit of driving battery cars in cities will be immediate: their quiet motors will reduce noise pollution and curb toxins like nitrogen oxide, NOX, a chemical compound spewed from diesel engines that’s hazardous to air quality and human health.

“In downtown Oslo, Stockholm, Beijing or Paris, the most immediate consideration is to improve air quality and the quality of life for the people who live there,” said Christoph Stuermer, the global lead analyst for PricewaterhouseCoopers Autofacts.

But electric cars aren’t as clean as they could be. Just switching to renewable energy for manufacturing would slash emissions by 65 percent, according to Transport & Environment. In Norway, where hydro-electric energy powers practically the entire grid, the Berylls study showed electric cars generate nearly 60 percent less CO2 over their lifetime, compared with even the most efficient fuel-powered vehicles.

As it is now, manufacturing an electric car pumps out “significantly” more climate-warming gases than a conventional car, which releases only 20 percent of its lifetime C02 at this stage, according to estimates of Mercedes-Benz’s electric-drive system integration department.

“Life-cycle emissions in electric vehicles depend on how much the car is driven in order to get to a point of crossover on diesels,” Ola Kallenius, the Daimler AG board member who will take over as CEO next year, said at the Paris Motor Show this month. “By 2030, the life cycle issue will improve.”

Some manufacturers have heeded calls to produce batteries in a more sustainable way. Tesla uses solar power at its Gigafactory for batteries in Nevada, and has plans for similar plants in Europe and Shanghai. Chinese firm Contemporary Amperex Technology Co. is also looking to power its future German plant with renewables.

“The topic of CO2 lifetime evaluations is starting to get more traction,” said Radics at Berylls. “Carmakers need to be transparent in this discussion to avoid unsettling buyers.”