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when is the best time to charge and electric vehicle?


Everything people think they know about the best time to charge electric cars is wrong

Current electric car owners tend to charge their cars at night. If that trend continues, it could create a big stress on the grid.
September 27, 2022

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A big ramp-up of electric vehicles (EVs), necessary to decarbonize the transportation system, could strain the electricity grid in the western United States, according to a new study. But shifting EV owners’ habits in terms of how they charge their cars will help avoid this problem.

The findings suggest a need to think more holistically about decarbonization: instead of conceiving of renewable energy infrastructure and EV charging infrastructure separately, policymakers need to consider the interplay between the two.

To reach that conclusion, researchers modeled the electricity needed for EV charging in 11 states across the western United States in 2035, based on plans for future electricity generation capacity. They calculated electricity needs if either 50% or 100% of the passenger vehicle fleet is electric at that time, and considered the impacts on the grid of varying charging habits.

Current EV owners—the early adopters—tend to be relatively wealthy and own their own homes. Many have installed home chargers and charge their vehicles overnight, when electricity is cheaper and it’s convenient to charge because their vehicles are not in use.

But if such habits continue as EV ownership becomes more widespread, they will put a massive strain on the electricity grid, the researchers report in Nature Energy. With 50% of the passenger vehicle fleet electrified, a preponderance of home charging could increase peak net demand for electricity by 25%. If all passenger cars were electric, the increase in demand would rise to 50%.

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The basic problem is that overnight charging is out of sync with the availability of renewable electricity, especially solar. So there are two options to meet a surge in electricity demand for overnight charging: more fossil fuel-based electricity generation, or more expensive battery storage to hold solar power from the daytime when it is generated to the nighttime when it is needed.

The solution is to instead encourage people to charge their EVs during the day, by providing convenient and easy to access charging stations at workplaces or other places where people park their cars during the day, the researchers say.

Peak net electricity demand can be up to 3.4 times greater if everyone charges their EV overnight compared to a scenario in which daytime charging is encouraged, the researchers calculated. Daytime charging would also save grid operators hundreds of millions or even billions of dollars in battery storage costs.

“Focusing on daytime charging to minimize grid impacts is the first major conclusion of this study,” the researchers write. “Daytime-charging scenarios benefit from their alignment with solar generation while overnight-charging scenarios miss that opportunity.”

The researchers also found that charging controls—basically timers that control the flow of electricity to chargers based on electricity rates, ostensibly to reduce demands on the grid—can backfire as EV adoption expands.

For example, it’s common for home chargers to kick in at 9 p.m. That makes sense while EVs are relatively rare. But if everyone has an EV and starts charging their car at the exact same time, that’s a big problem.

Similarly, workplace chargers often have controls designed to smooth out peaks in charging demand or grid emissions throughout the day. But as renewable generation ramps up, this leaves a lot of solar energy capacity untapped. “Uncontrolled workplace charging is well aligned with solar generation,” the researchers write—so just go ahead and juice up people’s cars when they plug them in on arriving at work.

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The results suggest that policymakers should revisit electricity rate structures as renewable generation and EV adoption increase, the researchers argue. They should also start planning and building convenient daytime charging options now, in order to shift EV drivers’ charging habits onto a path that is more sustainable in the long term.

“The build-out of new charging stations represents a powerful multi-year timescale form of charging control to improve the impacts of EV charging, support equitable widespread adoption, reduce emissions, support renewable integration and smooth the transition to a decarbonized future,” the researchers write.

Source: Powell S. et al.  “Charging infrastructure access and operation to reduce the grid impacts of deep electric vehicle adoption.” Nature Energy 2022.

Image: ©Anthropocene Magazine

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