Should You Charge Your EV at Home or at Work?

Laurance Yap
May 22, 2023
Whether you’re running your dishwasher or charging your EV, current electricity rates are lower late in the evening. A Stanford University study, however, suggest that this rate structure is outdated. Indeed, the study says that charging at work during the day would ease EVs' burden on the grid.
charging vehicle at home

A Million Electric Cars Charging in California

In February, cumulative sales of electric cars in California reached one million vehicles – meaning they now account for about 6 per cent of cars and light trucks on the road in the state. California has implemented aggressive targets to increase that number to five million by 2030 – increasing their penetration to 30 to 40 per cent. Almost all of those vehicles are currently charged at home overnight – but a new study from Stanford University suggests that may not be the best way to charge.

Home charging overnight is the most common form of charging for a number of reasons. First, it’s convenient, fitting in with most people’s daily routines. But it’s also inexpensive, as electrical utilities' time-of-use rates are lower overnight, which encourages consumers to move their electricity use to the nighttime. Whether you’re running your dishwasher or charging your EV, current electricity rates are lower late in the evening.

The Stanford study, however, suggest that this rate structure is outdated, reflecting a time before significant solar and wind power supplies came online in California. Nowadays, demand for power in the evening is actually much higher. Indeed, at a local level, if a third of homes in a neighborhood have EVs and most of the owners charge starting at 11 p.m. or whenever electricity rates drop, the local grid could become unstable.

“The findings from this paper have two profound implications: the first is that the price signals are not aligned with what would be best for the grid – and for ratepayers,” said Ines Azevedo, one of the paper’s co-authors, and associate professor of energy 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.”

In short: for a more sustainable future requiring less investment, California should encourage EV drivers to charge during the day rather than overnight.

charging cars in parking lot

Solar Power Greatest During the Day

California’s grid actually has excess electricity during late mornings and early afternoons thanks to its impressive solar capacity. If most electric cars charge during these times, that capacity would be used up instead of wasted. Daytime charging would relieve the pressure on the grid, and reduce the investment required to support more EVs on the road.

“We were able to show that with less home charging and more daytime charging, the Western U.S. would need less generating capacity and storage, and it would not waste as much solar and wind power,” said Siobhan Powell, lead author of the study. “And it’s not just California and Western states. All states may need to rethink electricity pricing structures as their EV charging needs increase and their grid changes.”

If most EVs continue to charge at night, however, the state might need to build more generators – likely powered by natural gas – or expensive energy storage on a large scale.

charging ev at work

Electric Car Charging Pricing Structure Needs to Change

Current pricing structures for electricity charge commercial and industrial customers higher fees based on what used to be considered peak electricity use. This disincentivizes companies from installing chargers that their employees can use. In reality, with the highest load on the grid now being at night, rate changes could help encourage more charging at work, and balance out the load.

“We encourage policymakers to consider utility rates that encourage day charging and incentivize investment in charging infrastructure to shift drivers from home to work for charging,” said the Ram Rajagopal, the study’s co-senior author and an associate professor of civil and environmental engineering at Stanford.

If you had access to charging at work, would you be more likely to consider an EV?

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