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Californians have been charging electric cars the wrong way, study says

A new study from Stanford researchers suggests that electric-car drivers who plug in while they’re snoozing at night should eventually alter their charging behavior to protect California’s electrical grid.

For years, EV owners have been told to charge in the middle of the night, when energy demand is lowest. Nighttime charging has been encouraged through lower utility rates and bulletins from state officials.

But in a study published Thursday in the scientific journal Nature Energy, a team of researchers concluded that most drivers should instead shift to daytime charging at work or at public charging stations. They said doing so would reduce the strain on the grid and limit how much money the state needs to invest in expanding the electrical system.

“Our best-case scenario was one that was dominated by a lot of workplace charging,” said Siobhan Powell, the lead author and a former Ph.D. student in mechanical engineering. “It matters a lot when you add demand in the future grid.”

The team of researchers at Stanford’s Precourt Institute for Energy found that daytime charging is preferable to nighttime residential charging for one major reason: solar panels.

California already generates an abundance of electricity from solar farms during sunny daytime hours. But it wastes a large portion of that energy because there’s nowhere to store it, so utilities must rely on electricity imported from other states and gas-fired plants when the sun goes down.

Researchers behind the study used computer models to project how increasing sales of electric cars could affect electrical demand in the Western US through 2035 — and beyond.

Without major investments in the grid and changes in charging habits, the study conclusions, charging for electric cars will significantly stress the grid when 30% to 40% of cars on the road are EVs. Only about 6% of cars and light trucks in California are electric today.

Powell said if drivers could shift their charging habits by 12 hours, and plug in between 8 am and 2 pm, it would significantly reduce that burden on the grid and limit costs.

Without that shift in behavior, the study projects more than 5.4 gigawatt-hours of energy storage would be needed to power vehicle-charging in the Western US at nighttime. That’s equivalent to the energy provided by five large nuclear power reactors.

Concerns about electric cars overtaxing the power grid are nothing new, though they have been a frequent source of misinformation as of late.

Electric cars were not a driving factor behind the power shortage that led California to repeatedly urge consumers to conserve to avoid rolling blackouts while a record-setting, 10-day heat wave earlier this month.

Some Republican and far-right critics had tried to link the crisis to Gov. Gavin Newsom’s policy requiring the state to ban the sale of most new gas-powered cars by 2035.

Data from the Energy Commission flatly debunks those claims. EV charging accounts for only 0.4% of overall energy load during peak hours on a typical summer day, from 4 pm to 9 pm, when the grid is most at risk of failure.

The Stanford study doesn’t suggest vehicle-charging is causing power shortages today, but it raises alarms about the long-term potential for problems as electric-vehicle sales surge.

But the authors said California and other Western states can prepare for the transition by investing in building more charging stations at workplaces and in other public locations. Otherwise, they said the state might be forced to rely on power generators fueled by natural gas.

California’s budget this year includes $3 billion to build more public charging stations, and the state recently received $56 million from the federal government for that purpose. A measure on the Nov. 8 ballot, Prop. 30, also proposes to raise taxes on wealthy people and pump the money into EV-charging stations and other projects to reduce greenhouse gas emissions.

Powell said the authors also recommend that utilities change their off-peak rate structures to encourage charging in the late morning and early afternoon. PG&E currently offers lower rates for EV charging from 12 am to 3 pm, and data from the Energy Commission suggests the vast majority of drivers charge at night.

The study states that existing utility-rate structures are a vestige from a time before the state had significant solar and wind power supplies, when demand threatened to exceed supply during the day

Ram Rajagopal, an associate professor of civil and environmental engineering and co-author of the study, said some drivers will always have a need to charge at home, especially those who don’t work outside the home.

But, he said, daytime charging outside the home will be cheaper and more efficient for the masses, particularly those who live in apartments without easy access to charging stations overnight.

“Daytime charging needs to be given the appropriate attention,” Rajagopal said. “You’re not maintaining gas pumps at home.”

Dustin Gardiner (he/him) is a San Francisco Chronicle staff writer. Email: dustin.gardiner@sfchronicle.com Twitter: @Dustingardiner

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