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

As electric vehicles hit the road around the country, hundreds of thousands of Americans are starting 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 anxiety. “

But as EV owners plug in their cars, there̵[ads1]7;s a looming problem: pressure on the power grid if most drivers continue to charge their EVs at night.

According to a new study by researchers at Stanford University, if electric car sales grow rapidly over the next decade — and most drivers continue to charge their electric cars at home — car charging could strain the power grid in the western United States, increasing peak net demand by 25 percent. 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 you need to know about EV charging is that there is nothing like filling a car with gas. Charging an electric car takes time – while the fastest chargers can charge an electric car battery to 80 percent in 20 to 30 minutes, most chargers are slower, taking anywhere from two to 22 hours to get a full charge. This means that around 80 per cent of electric car charging takes place at the owner’s home, overnight – when the driver does not need the car and can leave plenty of time for charging.

But that charging pattern is at odds with how electricity is increasingly produced. The greatest demand for electricity occurs in the evening, between 5pm and 9pm. People come home from work, turn on the lights, watch TV and do other activities that suck electricity. Solar panels, meanwhile, produce their energy in the middle of the day. The highest demand for electricity therefore occurs just when the solar energy has started to switch off for the day.

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

“When 30 or 40 percent of cars are electric, that’s going to have a significant impact on what we do with the grid,” said Ram Rajagopal, a professor of civil and environmental engineering at Stanford University and one of the study’s authors. Even if drivers wait until after rush hour and set their cars to charge at 11pm or later, they will be using electricity at precisely the time when renewable energy is not readily available. This can lead to increased carbon emissions and the need for more batteries and storage in the power grid.

One solution, say the researchers, is if more electric car owners switch to charging during the day, charge their cars at work or at public chargers. If electric cars are charged in the late morning and early afternoon, when the grid has excess solar energy that is not being used, there will be less pressure on the power system and less need for storage. According to the study, under a scenario where 50 percent of cars are electric, a shift from mostly home to a mix of home and work charging could almost halve the amount of storage needed on the grid. Adding workplace and public chargers has the added benefit of also helping renters or those who don’t own homes gain access to EVs.

Siobhan Powell, a postdoctoral fellow at ETH Zurich in Switzerland and the study’s lead author, says the time to plan for the expansion of public and workplace charging is now. “We’re not saying, ‘No more home charging’ or ‘limit home charging,'” she said. “We don’t want to be discouraged some charger because it is very important for adoption. But charging costs a lot of money, and we can make charging at work or in public just as convenient as it is at home.”

The authors also recommend changing electricity price structures to better encourage charging in the middle of the day. Currently, some utilities are offering super low electricity rates to consumers to charge their cars overnight. PG&E, for example, a utility in California, offers electric car owners electricity for 25 cents overnight between midnight and 7 a.m. and 36 cents between 07.00 and 14.00. Ideally, Rajagopal and Powell say, the cheapest prices should be in the middle of the day to encourage charging when the sun is out.

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

But he agrees that one of the advantages of electric cars is the flexibility when they can be charged. Switching to daytime charging will be useful, whether it’s by charging at home during the day (for those who work from home) or getting workplace chargers.

Policymakers need to “put chargers where the cars are during the day,” he said.

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