All the promotional videos and brochures on plug-in electric vehicles (PEVs) basically tell the same story: A happy homeowner comes home after a long day at work, parks her emissions-free car in the garage and, before going inside, plugs it into the nearby electrical socket to charge. It’s a scenario that’s playing out in homes across the country. There’s just one hitch. When the local utilities considered the amount of energy a home required a decade or more ago, most didn’t take into account electric vehicles. 

“When utilities were planning for what a typical electrical load for a house would look like, they were thinking about air conditioners, refrigerators and other appliances,” says Matteo Muratori, a transportation and energy systems engineer at the National Renewable Energy Laboratory. “They weren’t thinking about plug-in electric vehicles.”

Since more and more people are choosing to drive electric cars and charging them at home, Muratori created a computer simulation to explore the impact of in-home charging on the grid. His conclusion: Don’t expect any massive power outages anytime soon. “PEVs aren’t going to destroy the grid,” he says. 

Indeed, Muratori’s simulation, which he’s written about in a new paper, found that a plug-in market share of up to 3 percent, or about 7.5 million vehicles, won’t significantly impact the aggregate residential power demand. For perspective, there are roughly 700,000 PEVs on U.S. roads today. It won’t be until at least 2025 before U.S. roadways see more than 7 million PEVs, according to the Edison Electric Institute, a trade association.

But while there’s no immediate threat to the grid, Muratori cautions that in-home charging could still take its toll on electric system infrastructure by way of “uncoordinated charging.” Consider this scenario: Your next-door neighbor buys a plug-in electric vehicle. In a “Keeping Up with the Joneses” type of effect, this inspires another neighbor to buy one, then another and another until almost every homeowner on your block has a PEV. Each night these neighbors come home after work at around the same time and promptly plug in their electric vehicle to charge. This uncoordinated charging “will significantly increase the peak demand seen by distribution transformers and might require upgrades to the electricity distribution infrastructure,” according to Muratori’s paper.

Another way in-home charging could cause some wear and tear depends on which charging options households use: the less powerful Level 1 -- essentially an extension cord -- or the faster and more powerful Level 2 option. Much like uncoordinated charging, as more PEVs are added to a neighborhood and a higher charging power is adopted, “the distribution infrastructure might no longer reliably support the local electricity demand.” Muratori notes that the higher demand could shorten the expected life of a transformer.

So what’s the takeaway for state and local policymakers? From Muratori’s perspective, the report serves two purposes. The first is to remind stakeholders not to overlook this additional demand on the grid. PEVs should play a big role in future electric system planning. For instance, he says, utilities may want to think about how to incentivize some PEV owners to delay charging their vehicles when they get home at night. This will avoid the unintended consequences of uncoordinated charging. The second purpose is for policymakers to better understand the issue. Utilities may seek to design new rate structures to deal with the future influx of PEVs, he says. Elected leaders need to understand why utilities are doing so.  

Most important, though, future research should focus on understanding consumer behavior to determine charging requirements. “This is a new area for utilities,” says Muratori. “This will help them better anticipate tomorrow’s needs.”