Charging cars at home overnight is not the right choice

Charging cars at home overnight is not the right choice

The vast majority of electric vehicle owners charge their cars at home in the evening or overnight. We’re doing it wrong, according to a new Stanford study.

If regular evening or nighttime charging of electric vehicles shifts to daytime work at work as more cars go electric, that would cut additional costs for electricity systems, according to a new study from Stanford University. Credit: Amy Adams

In March, the research team published a paper on a model they created for charging demand that can be applied to a range of populations and other factors. In the new study, published Sept. 22 in Nature Energy, they applied their model to the entire western United States and examined the stress the region’s power grid will experience by 2035 as a result of growing EV ownership. In just over a decade, they found that rapid growth in the number of electric vehicles alone could increase peak electricity demand by as much as 25%, assuming continued dominance of residential and overnight charging.

To mitigate the high costs of all that new capacity to generate and store electricity, drivers should switch to day charging at work or public charging stations, which would also reduce greenhouse gas emissions, the researchers said. This finding has policy and investment implications for the region and its utilities, especially since California moved in late August to ban the sale of gasoline-powered cars and light trucks from 2035.

“We encourage policy makers to consider usage charges that encourage daily charging and incentivize investment in charging infrastructure to move drivers from home to work for charging,” said the study’s co-senior author, Ram Rajagopalan associate professor of civil and environmental engineering at Stanford.

In February, cumulative electric vehicle sales in California reached one million, accounting for about 6% of cars and light trucks. The state has targeted five million EVs on the road by 2030. When penetration reaches 30% to 40% of cars on the road, the network will experience significant stress without major investments and changes in charging habits, Rajagopal said. Building that infrastructure requires significant lead time and cannot be done overnight.

“We looked at the entire western region of the US because California relies heavily on electricity imports from the other western states. Charging electric vehicles plus all other electricity consumption impacts the entire Western region, given the interconnected nature of our power grid,” said Siobhan Powelllead author of the March study and the new one.

“We were able to show that with less home charging and more daytime charging, the western US would need less generating capacity and storage, and not waste as much solar and wind energy,” said Powell, a mechanical engineering PhD ’22.

“And it’s not just California and western states. All states may need to rethink their electricity pricing structures as their EV charging needs increase and their grid changes,” added Powell, who recently held a postdoctoral research position at ETH Zurich.

Once 50% of the cars on the road are powered by electricity in the western U.S.—about half of the population lives in California—more than 5.4 gigawatts of energy storage would be needed if charging habits continued their current course. . That is the capacity equivalent of 5 large nuclear reactors. A major shift to charging at work rather than at home would reduce the storage space required for EVs to 4.2 gigawatts.

Change incentives

Current usage times encourage consumers to switch electricity consumption to night time whenever possible, such as running the dishwasher and charging electric vehicles. This rate structure reflects the time before significant deliveries of solar and wind energy, when daytime demand threatened to outpace supply, especially in the late afternoons of summer.

Today, California has excess electricity during the late mornings and early afternoons, mainly due to its solar capacity. If most EVs charged during these times, the cheap power would be used instead of wasted. Alternatively, if most EVs continue to charge overnight, the state will need to build more generators — likely powered by natural gas — or expensive energy storage on a large scale. Electricity that goes first to a huge battery and then to an EV battery loses power from the extra stop.

On a local level, if a third of homes in a neighborhood have electric cars and most owners continue to charge at 11pm or when electricity rates drop, the local grid can become unstable.

“The findings of this article have two profound implications: the first is that the price signals are not aligned with what would be best for the network – and for taxpayers. The second is that it calls for investment in a charging infrastructure for where people work,” said Ines Azevedothe other co-senior author of the new paper and associate professor of energy science and engineering in the Stanford Doerr School of Sustainabilitywhich opened on September 1.

“We need to act quickly towards decarbonising the transportation sector, which is responsible for the bulk of California’s emissions,” Azevedo continued. “This work provides insight into how to get there. Let’s make sure we pursue policies and investment strategies that enable us to do this in a sustainable way.”

Another problem with electricity price design is that commercial and industrial customers charge high fees based on their peak electricity consumption. This can discourage employers from installing chargers, especially when half or more of their employees have electric cars. The research team compared different scenarios of charging infrastructure availability, along with different residential usage rates and commercial demand costs. Some tariff changes worsened the situation at grid level, others improved it. Nevertheless, a charging infrastructure scenario that encourages more daytime charging and less home charging offered the greatest benefits, the study found.

Rajagopal and Azevedo are also co-directors of the Bits & Watts Initiative at Stanford’s Precourt Institute for Energy. Other co-authors of this study are: Gustavo Cezar, PhD student and a staff engineer at Stanford’s SLAC National Accelerator Laboratory; and Liang Mindirector of the Bits & Watts initiative.

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

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