5 charts that shed new light on how people charge EVs at home

Thanks to a combination of government policies and decreasing costs, battery-powered cars will soon be commonplace in U.S. garages — the Biden administration has set an ambitious goal for half of all new cars sold in 2030 to be electric. According to the U.S. Department of Energy, 80 percent of EV charging happens at home, so figuring out how all of these cars will affect their owners’ electric bills — and the utility grids they’re connected to — will become a far more pressing matter.

One place to look for insights on these questions is Austin, Texas. Over the past decade, hundreds of households have volunteered to provide nonprofit research organization Pecan Street access to second-by-second data on how much electricity their air conditioners and appliances are using, how much their rooftop solar systems are generating — and the charging patterns of their electric vehicles.

Since the federally funded data-collection project started in 2011, the more than 100 EVs in the program have shifted from early power-sippers like the Chevy Volt and Nissan Leaf to newer models with much bigger batteries from automakers like Tesla, Ford and BMW. All have remained under the owners’ direct control, providing a uniquely long-running and granular view of how EV owners’ charging behaviors have changed over time and under circumstances such as the coronavirus pandemic and last year’s winter grid emergency in Texas.

This extensive trove of data allows Pecan Street to game out a number of propositions for the future of home EV charging.

“There are no one-size-fits-all solutions to when to charge,” Pecan Street CTO Scott Hinson said during a recent webinar summarizing the group’s data over the past decade. ​“There are times we’re going to want to push charging toward [periods of higher] solar production. There will be other times we want to push charging away from certain times,” such as when grids are facing extreme-weather-related stresses.

A growing number of utilities are enlisting customers in managed-charging programs that use time-varying electricity rates or remote-control signals to chargers or to EVs themselves to serve these tasks. A 2021 report from the Smart Electric Power Alliance trade group highlighted the ways these programs are preparing to ​“avoid distribution upgrade bottlenecks and mitigate unnecessary stresses” as the number of EVs on U.S. roads grows to a predicted 25 million by 2030.

“Fortunately, the vehicles don’t care” that much about having their charging delayed by a few hours per day, Hinson said. That gives EV owners and utilities quite a bit of leeway in altering charging patterns to prevent grid overloads, absorb more renewable energy, or even balance out the increasing electricity demand from homes that are switching from fossil-fueled to electric heating and cooking, he said. Here are some key findings from Pecan Street’s analysis that illustrate why home EV charging offers such flexibility and how useful it could be.

EV charging tends to come in sips, rather than gulps

One of the main reasons Hinson sees a lot of flexibility in the home EV charging mix is because, for the most part, those EVs aren’t actually consuming that much electricity each time they charge.

This chart of Pecan Street data of typical EV charging cycles illustrates the point. For the most part, ​“charges are very frequent and in relatively small amounts,” Pecan Street data analyst Cavan Merski said.

While the percentage of charging sessions that exceed 20 kilowatt-hours in draw has grown over time as EV batteries have gotten bigger, about 70 percent of charging sessions use less than 10 kilowatt-hours per plug-in, and a sizable portion of charging sessions are less than 5 kilowatt-hours, he noted.

Left to their own devices, people tend to charge at very unpredictable times 

Merski emphasized that Pecan Street’s volunteers aren’t enrolled in managed charging programs, time-of-use pricing rates or other utility structures that would alter how much they pay for charging their EVs from one hour to the next. That means they don’t have any incentives to charge at any particular time — and not surprisingly, that’s led to a fairly unpredictable range of typical charging patterns, as this chart shows. 

At the same time, customers who do sign up for time-varying prices have been fairly consistent in shifting when they charge to take advantage of those price differentials, Hinson said. Back in its early days of EV testing, Pecan Street ran a time-of-use charging pilot that encouraged people to charge overnight, when wind energy is usually cheap and plentiful, and most customers who volunteered for it did end up charging less during peak summer daytime hours and charging more overnight, he said.

In fact, even though the pilot ended years ago, ​“there are people whose habits were formed…and they still follow those same charging behaviors,” he said, even though ​“there’s no economic benefit for them to do that.”

Changing when vehicles are charged can save a lot of money — if the rates are right 

That’s not to say the economic benefits of altering charging times don’t exist, Hinson added. To the contrary, the difference between charging lots of EVs at the peak of grid demand and charging them when power is cheap and plentiful can be dramatic—if, that is, the value of altering that charging can be translated to what EV owners are paying for their power.

The following chart uses data from a previous Pecan Street report to illustrate that point. It compares the typical pattern of people charging their EVs as soon as they return home from work in the afternoon to a managed-charging profile that delays charging to overnight hours. It then overlays a typical wholesale market price graph from a grid-stressed summer day on the system operated by Texas grid operator ERCOT.

The difference in the wholesale market cost for the electricity used by these two distinct charging schemes is quite significant, Merski said. 

Unrestrained charging during post-work hours would increase the cost of the wholesale power purchased to supply those EVs by 20 percent, compared to a ​“status quo” charging pattern that more evenly spread that charging through the late evening and early morning. Conversely, delaying charging until close to midnight and through the early morning could reduce the cost of wholesale power purchases by nearly 40 percent, he said.

Hinson emphasized that these results from ERCOT may differ from region to region. ​“The point, again, is that the flexibility is there to do it.” 

There’s a catch to this equation, however: Almost no EV owners are actually exposed to the drastic peaks and valleys of wholesale electricity markets. Instead, most utility customers are on flat retail rates. Even the various EV charging time-of-use rates that are available don’t have the same drastic ups and downs, since that could expose customers to massive price spikes they may not be prepared to pay.

Still, Merski noted that ​“as the grid changes, as homes electrify, these price signals will change.” The challenge, he said, is to design rate structures and programs that maximize both the value of flexible EV charging and the willing participation from EV owners — a key issue for utilities that see customer buy-in as the biggest impediment to successful charging programs, according to SEPA’s 2021 survey.

People could balance the demands of all-electric homes with their EVs — if technology and regulations allow it

Beyond helping the power grid balance supply and demand, flexible EV charging could also help with far more local grid-balancing tasks, Hinson said — including balancing demand at the household level.

That point of interconnection between homes and their local grid circuits and transformers becomes far more important as homes shift from fossil-fueled furnaces, water heaters, stoves and clothes dryers to electric heat pumps and appliances, he said.

Pecan Street research indicates that about three-quarters of U.S. homes, or about 48 million, are likely to need to upgrade their electrical panels to make the switch from fossil-fueled to all-electric heating and appliances. That can cost thousands of dollars and take weeks of work with electricians and permitting agencies. If utilities need to upgrade their local grid equipment to support that home’s increase in electrical load, those costs and wait times can increase significantly.

That makes EVs a potentially pivotal player in balancing the ups and downs of electricity use in homes. ​“You can really flatten out that home load, and fill in the gaps, and now the home becomes a lot easier for the utility to serve electricity to,” Hinson said. ​“And you mitigate some of the needs for panel upgrades if systems like this are allowed.”

“We’ve designed systems to do this and demonstrated this,” Hinson added. The chart below shows how one such EV-charging-balanced home in Pecan Street’s volunteer group was able to modulate its charging to balance out overnight changes in electric heating loads.

To be clear, EVs are the biggest of the new electrical loads going into these equations, Pecan Street’s research shows. But they’re also ​“the most flexible load we’ve ever added to a residential structure,” Hinson said. Only electric water heaters have as much flexibility in terms of how long they can be left off or how far in advance they can charge up, he said, and they don’t carry nearly the same stored energy punch as do EV batteries.

Even more flexibility could be opened up with vehicle-to-home charging, which allows the electricity stored in EV batteries to power the home itself, he noted. The most well-publicized concept is using EVs to provide backup power to homes during outages. Pecan Street studied the potential for batteries of different capacities to provide power for essential home systems during the weeklong blackouts that afflicted much of Texas during last year’s Winter Storm Uri. It found that the largest batteries now available could have kept some homes running through much of the longest outages.

All of these potential uses need a lot of work on the technical and regulatory side to become a reality, however, he said. ​“Right now, there’s this very slow race to a solution,” he said. One potential roadblock is a lack of common standards to allow EVs, charging equipment and home electrical equipment to interoperate smoothly. ​“We need to fix that, and we need to fix that in a hurry.”