Study finds charging EVs during the day is better for the grid

Study finds charging EVs during the day is better for the grid
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Charging electric vehicles from home at night is not the way to go, according to a new study from Stanford University. Instead, it says we should be charging during the day at work or at public charging stations.

A research team from Stanford University published the paper which modeled electric vehicle charging demand for the entire western United States. He examined the stress the region’s power grid may be subject to by 2035 as the electric vehicle market grows.

The researchers found that peak electricity demand will increase by up to 25% if electric vehicle charging continues to be done primarily from home at night.

But they concluded that the increase in electricity demand costs caused by electric vehicles could be limited. Drivers can help achieve this by charging at work or at public charging stations during the day.

Such a move would also help reduce greenhouse gas emissions from all new capacity to generate and store electricity.

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Charging electric vehicles during the day wastes less solar and wind energy

“We look at the entire western region of the US, because California relies heavily on electricity imports from other western states. Electric vehicle charging plus all other uses of electricity have consequences for the entire Western region given the interconnected nature of our electrical grid,” said Siobhan Powell, a Stanford mechanical engineering doctoral candidate and lead author of the study.

“We were able to show that with less charging at home and more charging during the day, the western US would need less generation and storage capacity, and not waste as much solar and wind energy.

“And it’s not just about California and the western states. All states may need to rethink electricity pricing structures as their EV charging needs grow and their grid changes.”

Cumulative electric vehicle sales in California reached 1 million in February, accounting for about 6% of cars and trucks. However, California is targeting five million electric vehicles on the road by 2030.

But when the level of penetration of electric vehicles reaches 30% or 40%, the network will experience significant stress.

Big investments and changes in charging habits can help prevent this. Infrastructure investment decisions need to be made now to avoid this, as development requires significant lead time.

If EV owners charge at night, more batteries will be needed

Stanford researchers say more than 5.4 GWh of energy storage will be needed when 50% of cars on the road are electric vehicles. This is the case if nightly charging habits remain as they are.

However, if habits change to start charging at work during the day, the storage need would only be 4.2 GW.

“We encourage policymakers to consider utility rates that encourage daily charging and incentivize investment in charging infrastructure to shift drivers from home to work to charge,” said Ram Rajagopal, study co-lead author and associate professor of engineering. civil and environmental at Stanford. .

Time-of-use electricity rates encourage consumers to shift their electricity usage to nighttime hours whenever possible. This is true in California and elsewhere.

But these rates are a reflection of a time before the widespread deployment of wind and solar power, when demand threatened to outstrip supply during the day, especially in the late afternoon during the summer.

California has lots of sunlight, and this solar energy

Today, however, California has a surplus of electricity during the late morning and early evening hours thanks in large part to its massive solar capacity.

Drivers can make better use of cheaper solar-generated electricity if most electric vehicles are charged during these times.

“This paper’s findings have two profound implications: The first is that pricing signals are not aligned with what would be best for the network and for taxpayers,” said Inés Azevedo, another co-senior author and associate professor on the new paper. in science and engineering at the Stanford Doerr School of Sustainability.

“The second is that it calls for considering investments in a charging infrastructure for where people work.

“We need to move quickly toward decarbonizing the transportation sector, which accounts for the majority of emissions in California. This work provides information on how to get there. Let’s make sure we apply investment policies and strategies that allow us to do it in a sustainable way.”


Siobhan Powell, Gustavo Vianna Cezar, Ram Rajagopal,
Scalable probabilistic estimates of electric vehicle charging given observed driver behavior,
Applied Energy, Volume 309, 2022,
118382, ISSN 0306-2619,

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