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Stanford study warns against charging electric cars at home overnight

[Mar. 28, 2023: Mark Golden, Stanford University]

The vast majority of electric vehicle owners charge their cars at home in the evening or overnight. (CREDIT: Creative Commons)

A new study from Stanford University has found that the vast majority of electric vehicle (EV) owners are charging their cars at home in the evening or overnight, and this could be costing the electricity grid a significant amount. The study has recommended that the practice should change, with more EV owners charging their cars during the day at work or at public charging stations.

The study, which was published in the journal Nature Energy, examined the stress that the western United States’ electric grid will come under by 2035 from growing EV ownership.


It found that if rapid EV growth continued with a continued dominance of residential, nighttime charging, peak electricity demand could increase by up to 25% in just over a decade. However, if more people shifted their charging habits to daytime at work or public charging stations, it would reduce greenhouse gas emissions and prevent the need for the extra costs of generating and storing electricity.

The study’s co-senior author, Ram Rajagopal, an associate professor of civil and environmental engineering at Stanford, said: “We encourage policymakers to consider utility rates that encourage day charging and incentivize investment in charging infrastructure to shift drivers from home to work for charging.” The implications of the study could also affect utilities in the region, especially since California moved in late August to ban sales of gasoline-powered cars and light trucks starting in 2035.


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The study considered the entire Western US region, as California depends heavily on electricity imports from other western states. “EV charging plus all other electricity uses have consequences for the whole Western region given the interconnected nature of our electric grid,” said Siobhan Powell, lead author of the study. She added: “All states may need to rethink electricity pricing structures as their EV charging needs increase and their grid changes.”

Currently, California has excess electricity during late mornings and early afternoons thanks mainly to its solar capacity.


The researchers suggest that if most EVs were charged during these times, then the cheap power would be used instead of being wasted. However, if most EVs continue to charge at night, then the state will need to build more generators, likely powered by natural gas, or invest in expensive energy storage on a large scale. Electricity going first to a huge battery and then to an EV battery loses power from the extra stop.

Overview of the methods for evaluating grid impacts and modelling EV charging demand. (CREDIT: Nature Energy)

Another issue with electricity pricing design is that commercial and industrial customers are charged large fees based on their peak electricity use, which can disincentivize employers from installing chargers, especially once half or more of their employees have EVs.


The study compared several scenarios of charging infrastructure availability, along with several different residential time-of-use rates and commercial demand charges. Some rate changes made the situation at the grid level worse, while others improved it. Nevertheless, a scenario of having charging infrastructure that encourages more daytime charging and less home charging provided the biggest benefits.

Profile of aggregate EV charging demand illustrated for each infrastructure and control scenario. (CREDIT: Nature Energy)

The study also found that, once 50% of cars on the road in the Western US are powered by electricity, more than 5.4 gigawatts of energy storage would be needed if charging habits follow their current course. That’s the capacity equivalent of five large nuclear power reactors. A big shift to charging at work instead of home would reduce the storage needed for EVs to 4.2 gigawatts. However, building the necessary infrastructure requires significant lead time and cannot be done overnight.


Ines Azevedo, the study’s other co-senior author and associate professor of energy science and engineering in the Stanford Doerr School of Sustainability, said: “We need to move quickly toward decarbonizing the transportation sector, which accounts for the bulk of emissions in California. This work provides insight on how to get there.

The researchers noted that while policies to incentivize daytime charging are important, so too is investment in charging infrastructure at workplaces and public areas. Encouraging employers to install chargers and making public charging stations more widely available could help to facilitate the transition to daytime charging.

This shift could also have a significant impact on greenhouse gas emissions. The researchers found that if EVs are charged during times when solar and wind energy are most abundant, this could help to reduce the need for natural gas-fired power plants and other sources of carbon-intensive electricity.

The study has important implications for policymakers, particularly in California where the transition to EVs is expected to accelerate in the coming years. In addition to the ban on gasoline-powered cars and light trucks by 2035, the state has also set a target of five million EVs on the road by 2030.

As the researchers note, meeting these targets will require significant investments in charging infrastructure and changes in consumer behavior. By encouraging more daytime charging and incentivizing employers to install chargers, policymakers can help to ensure that the transition to EVs is as sustainable and cost-effective as possible.


Overall, the study highlights the importance of considering the grid-level impacts of EV charging and the need for coordinated policy and investment strategies to ensure that the transition to EVs is successful. With the right policies and investments, the researchers argue, it is possible to create a more sustainable and resilient energy system that benefits both ratepayers and the environment.

This work was funded by the California Energy Commission, by the National Science Foundation, and by the Bits & Watts Initiative with support from Volkswagen.

For more science news stories check out our New Innovations section at The Brighter Side of News.


Note: Materials provided above by Stanford University. Content may be edited for style and length.


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