It is common to charge electric vehicles at night. That will be a problem.


As electric vehicles hit the road across the country, hundreds of thousands of Americans are beginning to learn the ins and outs of car charging: how to install home chargers, where to find public charging stations, and how to avoid the dreaded “range fear.” ”

But when EV owners plug in their cars, there’s a looming problem: strain on the grid if most drivers keep charging their electric cars overnight.

According to a new study from researchers at Stanford University: If EV sales grow rapidly over the next decade — and most motorists continue to charge their EVs at home — vehicular charging could put a strain on the electrical grid in the western United States, reducing net energy costs. demand would increase by 25 percent during peak hours. That could be a problem as the West struggles to keep the lights on amid heat waves and rising demand for electricity.

The first thing to know about electric vehicle charging is that it is nothing like filling a car with gasoline. Charging an electric car takes time – while the fastest chargers can charge an EV battery to 80 percent in 20 to 30 minutes, most chargers are slower, taking between two and 22 hours to fully charge. That means around 80 percent of EV charging happens at the owner’s home overnight – when the driver doesn’t need the car and can leave plenty of time for charging.

The mass adoption of electric vehicles will change everything we know about cars – from driving them to repairing them. But the shift will be bumpy. (Video: Lee Powell/The Washington Post, Photo: Brian Monroe/The Washington Post)

But that charging pattern is at odds with how more and more electricity is generated. The greatest demand for electricity occurs in the evening, between approximately 5:00 pm and 9:00 pm. People come home from work, turn on the lights, watch TV, and do other activities that use power. Solar panels meanwhile produce their energy in the middle of the day. So the highest electricity demand occurs just when solar energy starts working turn of for the day.

In the Stanford study, researchers modeled the charging behavior of residents of 11 western states and analyzed how that behavior would affect a power grid that is increasingly switching to renewables and other clean energy sources.

“Once 30 or 40 percent of cars are EVs, it will have a significant impact on what we do with the electrical grid,” said Ram Rajagopal, a professor of civil and environmental engineering at Stanford University and one of the authors of the study. Even if drivers wait until after rush hour and charge their cars at 11pm or later, they are using electricity right when renewable energy is not readily available. This could lead to increased CO2 emissions and a need for more batteries and storage in the electricity grid.

One solution, the researchers say, is if more EV owners switch to daytime charging, charging their cars at work or at public chargers. Charging electric cars in the late morning and early afternoon, when the grid has excess solar capacity that is not being used, will reduce the strain on the electricity system and reduce the need for storage. According to the study, in a scenario where 50 percent of cars are electric, a shift from primarily home charging to a mix of home and work charging could nearly halve the amount of storage space required on the electrical grid. Adding workplace and public chargers has the added benefit of helping tenants or those who don’t own homes have access to EVs.

Siobhan Powell, a postdoctoral researcher at ETH Zurich in Switzerland and the study’s lead author, says now is the time to make plans for expanding public and workplace charging. “We’re not saying, ‘Don’t have any more home charging’ or ‘limit home charging,'” she said. “We don’t want to discourage each charging because that’s really important for adoption. But there’s a lot of money going into charging, and we can make charging at work or in public as easy as at home.”

The authors also recommend shifting electricity price structures to better encourage mid-day charging. Right now, some utilities are offering super low electricity rates to consumers to charge their cars overnight. PG&E, for example a California utility, offers EV owners electricity for 25 cents at night between midnight and 7am and 36 cents between 7am and 2pm. Ideally, say Rajagopal and Powell, the cheapest fares should be mid-day to encourage charging when the sun is out.

Gil Tal, the director of an electric vehicle research center at the University of California at Davis who was not involved with the paper, said current EV owners shouldn’t worry about their charging patterns. “We don’t have to step on the brakes when using electric cars,” he said. As more clean energy and storage is added to the grid, many of these problems will be solved, he says.

But he agrees that one of the advantages of EVs is the flexibility of when they can charge. Switching to day charging will be helpful, whether that be by charging at home during the day (for those who work from home) or by providing chargers in the workplace.

Policymakers should “place the chargers where the cars are during the day,” he said.

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