The efficiency of electric cars is maximized when they are shared and charged intelligently. To achieve this, it is essential to have real-time information on electricity prices and CO₂ emissions. This is demonstrated by a study from Empa and the University of Geneva (UNIGE), based on comprehensive data from carpooling and published in
Environmental Science & Technology.
Reducing greenhouse gas emissions related to passenger transport requires more electric vehicles and fewer cars on the road. Carpooling represents significant potential here. But not all charging is equal: depending on the time of day, the electricity mix (and thus the CO₂ emissions associated with electricity production) as well as electricity prices vary. By analyzing data from approximately 1.5 million Mobility users, scientists from Empa and UNIGE studied electric car charging based on induced emissions.
Most people in Switzerland, the country of the study, are unaware of either the actual electricity price or the associated CO₂ emissions at the time of charging.
Their results show that it is rarely possible to benefit from both low prices and reduced emissions simultaneously. Over the year, optimizing only costs allows for average savings of 21% but results in a fivefold increase in CO₂ emissions. Conversely, climate-optimized charging reduces emissions by up to 82% but increases costs by nearly 27%. "One of the main challenges is reconciling cost objectives and climate objectives. Ideally, environmentally friendly charging should also be financially attractive," explains Sven Eggimann, a researcher at Empa and author of the study.
The need for dynamic pricing and real-time information
The study's calculations are based on electricity prices that vary hourly. But if only simplified pricing structures, simply distinguishing between day and night, are implemented, smart charging becomes difficult to apply. "Most people in Switzerland are unaware of either the actual electricity price or the associated CO₂ emissions at the time of charging," emphasizes Elliot Romano, a researcher at Empa and scientific collaborator at the Institute of Environmental Sciences at UNIGE. To enable climate-friendly or cost-optimizing charging strategies, users need real-time information, ideally accessible via smart meters.
"In countries like Denmark, these drivers can check the current electricity price through an app and consciously choose their charging times," adds Sven Eggimann. "This system works, but no one wants to manage this manually in the long term." Hence the interest in automated solutions, capable of adapting to individual preferences.
For charging during low-emission periods to become economically attractive, appropriate incentives are also needed, such as electricity rates indexed to carbon intensity. Simulations conducted at Empa show that with a dynamic CO₂ price of about 30 cents per kilogram of CO₂ equivalent on average (equivalent to a tax of 72 cents/100 km, or about 100 CHF per year for a standard vehicle in Switzerland), it becomes possible to reconcile climate-friendly charging and cost optimization. But only if the rates truly reflect the emissions associated with electricity consumption.
"Ideally, charging should be based on voluntary approaches, supported by incentives," specifies Elliot Romano. "This could take the form of preferential rates or reserved parking spots during low-emission periods." Otherwise, access to charging could be restricted, or less flexible behaviors could be subject to additional fees.
More frequent use during the day, but an environmentally friendly solution
Shared vehicles are used more frequently and especially during the day. They therefore require more charging at night, a period when the electricity mix is often less favorable from a climate perspective. Yet, according to Sven Eggimann, the emissions and overall costs of their charging differ only slightly from those of private vehicles. "Shared cars are used more intensively, but thanks to short charging breaks and the growth of fast-charging infrastructure, there are still enough opportunities to prioritize low-emission periods."
Carpooling, on the other hand, offers an even more promising perspective: a substantial reduction in the vehicle fleet. With 25% fewer cars in Swiss cities, the pressure on winter electricity supply could be significantly alleviated. "Carpooling requires less energy overall, since there are fewer vehicles on the road," explains Elliot Romano. "Even if the total number of kilometers traveled each year remains comparable to that of private cars, an increasing share of trips is made via other modes of transport in addition to shared vehicles. This helps to relieve the energy system as a whole."
The structural challenge of winter
To achieve a sustainable transition in the transport sector, electric vehicles cannot be considered independently of optimizing their charging. "This requires both regulatory and technical advances," emphasizes Sven Eggimann. "In the long term, the goal is clear: to have a charging infrastructure capable of automatically guiding users toward low-cost and low-emission periods, without requiring constant intervention."
If Switzerland relies more on electric cars in the future, the energy system will need to adapt profoundly. Even with optimized charging times and a reduced vehicle fleet thanks to carpooling, the additional electricity demand related to the electrification of private mobility will remain high, with a simulated winter deficit of about one terawatt-hour per month by 2050.
"This seasonal supply deficit cannot be filled solely by more batteries or by simply shifting charging schedules," warns Elliot Romano. "Electrification is therefore only part of the solution. Those who truly want to act for the climate also rely on carpooling, public transport, and, more broadly, on reducing the use of private cars."