California needs to charge electric vehicles during day, not night, to save grid, study says

A charging plug inserted into an electric vehicle
Stanford researchers found California will need a drastic increase in public and workplace infrastructure to support daytime charging for electric vehicles. Above, an EV at a charging station at Chula Vista City Hall.
(Rob Nikolewski / San Diego Union-Tribune)
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To successfully ramp up Californians’ reliance on electric vehicles over the next decade without overtaxing the state’s power grid, more drivers will need to charge their cars during the day, a Stanford study has found.

The state needs to drastically increase and improve public and workplace charging infrastructure that supports daytime charging, which can better utilize solar energy, according to the study, published this week in the scientific journal Nature Energy. Increased charging availability will also make electric vehicle ownership more accessible, the study found.

“If we focus a lot of attention on building out a big public charging network wherever people are during the day — at work, in public places … that will be a big win for the grid,” said Siobhan Powell, the lead researcher on the study.


The study, part of Stanford’s Bits & Watts Initiative, which integrates researchers with leaders of the energy industry, used modeling to forecast EV charging demands in 2035 across 11 Western states, including California. Projecting that both EV use and renewable energy reliance will increase over the next 13 years, the team found that improvements in public charging infrastructure — reducing dependence on nighttime power-ups — could ensure that plug-in vehicles have the least effect on the power grid.

EV owners are likely to be in high-income brackets and to charge their batteries at home overnight — often taking advantage of incentives offered by power companies to do so. If those charging habits persist as EV use greatly expands, peak electricity demand could be increased by up to 25%, the researchers found.

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Though a shift to more daytime charging is not expected to stem the increase in demand, it would move the peak to when California has historically had excess capacity on the grid, thanks to solar power. This shift could also help the West prepare for high stress on its electricity system during heat waves, which continue to push the grid to its limits.

By 2035, the year California has committed to ending the sale of new fossil-fuel-burning vehicles, increased capacity will make daytime solar energy cheaper, said Ram Rajagopal, co-author of the study and an associate professor of civil and environmental engineering and electrical engineering at Stanford.

“Workplace charging, in particular, happens to line up really well with solar generation,” added Powell.

Scaling up stations that support daytime charging would “improve the impacts of EV charging, support equitable widespread adoption, reduce emissions, support renewable integration and smooth the transition to a decarbonized future,” the study found.


A trip from the Bay Area to Southern California highlights the miserable state of the charging network available to many electric vehicle drivers.

Sept. 22, 2022

“Building out a lot of public daytime charging will be beneficial for the grid but also can help bring charging access to more people,” Powell said.

She acknowledges that the state needs “millions more” charging stations to reach its goal of 5 million zero-emission vehicles on the road by 2030 — almost five times the current amount. But she’s hopeful that the research can help shape policy about how and where the stations are implemented.

The findings do not mean people should stop charging EVs at home and overnight if that works best for them, Powell said, because the priority should be encouraging the use of lower- or zero-emission vehicles. But she would like to see the study influence policymakers or companies that build charging stations. That way, “it can be the convenient choice for people to charge at that time when it’s better for the grid,” Powell said.

Rajagopal, who is director of the Stanford Sustainable Systems Labs, said it could help to have people start to get into the habit of charging during the day.

“If we get people used to the right behavior now, then when that transition completes, we will already be doing the right thing,” he said.