For electric car users, minimizing costs of charging and minimizing emissions are in conflict, a new analysis reveals. For car-share users, it can be even more difficult to align charging times with low-carbon electricity – but advancing technology and smart public policy can bridge the gap.
Electric vehicles (EVs) and car sharing “play complementary roles in the transition toward sustainable mobility,” says study team member Elliot Romano, a renewable energy systems scientist at the University of Geneva. “EVs address the energy source of transport, while car sharing targets the number of vehicles on the road.”
Many studies of the climate impact of EV charging use a fixed value for electricity-related emissions. But the mix of energy sources used to generate electricity varies from hour to hour throughout the day.
So does the price of electricity. And EV owners typically use price to decide when to charge their car.
In the new study, Romano and his colleagues calculated the cost, emissions, and grid consequences of EV charging in Switzerland, modeling a nationwide fleet of electric cars depending on different charging behaviors and private versus shared ownership models.
Always charging at the cheapest time reduces charging costs by 21% compared to prioritizing low emissions, the researchers report in the journal Environmental Science & Technology. By contrast, always charging at the most climate-friendly time can reduce emissions by up to 82%.
“At first glance, this seems counterintuitive, one might expect cleaner electricity to be cheaper, not more expensive,” says Romano. The pattern reflects the peculiarities of Switzerland’s electricity system – when prices are high, the country tends to generate and export hydropower; when prices are low, the country imports fossil-generated power from neighboring countries. But a similar mismatch has been seen elsewhere.
“The balance between saving money and cutting emissions varies by country, and the trade-off becomes sharper when energy prices and carbon signals don’t move in sync,” Romano says.
What’s needed instead is a system where it pays to charge your car in the most climate-friendly way.
To help accomplish this, the researchers recommend a dynamic carbon tax on electricity. Just 30 cents per kilogram of carbon dioxide would align price and emissions signals, the team found – as long as the carbon price reflects real-time emissions from electricity generation.
Such a policy would need to be combined with technology like smart meters and apps to give the public real-time updates on the electric grid. “Transparent information is key: consumers need to know not only what they’re paying for electricity, but also how clean that electricity is at any given moment,” says Romano.
Additionally, as more solar power is added to the grid in the future, cheap electricity and clean electricity will likely coincide to a greater extent.
Car sharing adds another wrinkle to the problem. The researchers used a database of 1.5 million car-sharing reservations in Switzerland to understand when and where shared cars tend to be used and charged.
Shared vehicles spend more time on the road than private vehicles, especially during the day. So they’re likely to need to be charged at night, when electricity tends to be more carbon-intensive. Still, short charging sessions throughout the day kept total emissions and costs of charging comparable to private vehicles, the researchers found.
In the future, fast-charging infrastructure will make it easier to optimize for both emissions and costs.
The shift to electric vehicles requires investments in the electric grid, the analysis also showed. If all private cars were electric in 2050, Switzerland would face an electricity shortfall of 1.3 terawatt hours per month during the winter. Even with optimized charging times and with car sharing reducing the number of vehicles on the road by 25%, the shortfall would still be 1.0 terawatt hours. More research is necessary to figure out how to ensure an adequate electricity supply while keeping emissions minimal, Romano says.
Source: Romano E. et al. “Emission-Responsive Charging of Electric Cars and Carsharing to Improve the Security of Electricity Supply for Switzerland.” Environmental Science & Technology 2025.
Image: © Anthropocene Magazine
