EVgo solar canopy EV charging station
A Chevy Bolt fills up at a DCFC station complete with a solar canopy.

Yes, if being green is your thing, it DOES matter when you plug your electric vehicle in

A Chevy Volt charges with solar-generated electricity.

editor's blog logoI have long been very interested in the question of how much it (does not) matter(s) when one plugs one’s electric vehicle in, especially if, like I do, I have the choice of plugging into a home solar system.

I have long preferred, when possible, to plug my electric car in during the day, ideally during a (really) sunny day, so that I know that as close as is possible to 100% of the electricity flowing into my EV’s battery is local, solar-generated solar electricity.

Countless people have made fun of me, claimed it doesn’t matter when I plug in my EV, even criticized me because they don’t seem to think it’s worth it to pay attention to where the actual electricity filling up my EV’s battery is truly coming from.

Some have claimed that solar offset — where a solar system produces more electricity than is being drawn from the local electric box into which it is tied and sends this extra electricity out into the grid to be used for things other than charging an EV — is equally as good as plugging in an EV when 100%, or a high percentage, of the electricity flowing into that EV’s batteries is being produced by a local solar system.

Sights like this one, near Sondershausen, Germany, where wind turbines and solar farms are located together, are common in Germany. [Photo by Christof Demont-Heinrich]
In fact, as it turns out, these people are wrong: It does matter when you plug in your EV, and, at least in some — perhaps many — cases, solar offset is not as green as plugging your EV in when your local solar system is producing electricity.

This, at least, if your primary goal is greening the world, and that is, has been, and always be, MY primary goal vis-a-vis solar-charged driving, EV + PV, or Driving on Sunshine.

It does indeed matter when you plug your EV in, according to a new study conducted by a team of researchers at MIT. In trying to assess the comparative green impact of when EV owners (do not) plug in their EVs, the team examined key questions and variables that, up to now, had, in my view, inexplicably been ignored by other researchers interested in the green impact of EVs.

Among the key questions and variables the MIT team at the MIT Energy Initiative zeroed in on ==>

  1. What is the specific electric grid mix in a given location — how much of the electricity is produced via renewables, fossil fuels, nuclear, etc.?
  2. What is the specific grid mix in a given place at a GIVEN, specific time of day?
  3. What are some of the specific differences in local, state, regional grid mixes AND at what time of day does the grid mix look like A, B or C in terms of its renewables to non-renewables mix?
  4. What are some of the, as it turns out, VERY BIG differences in various local, state, regional electric grid mixes, both across longer AND shorter periods of time, including REAL time?

New solar panels being installed in Summer 2020 in Littleton, Colo. [Photo by Christof Demont-Heinrich]
In fact, HUGE differences exist in both local grid mix and grid mix according to the time of day that one plugs in. Thus, for instance, in much of California, it is often much greener to plug your EV in during the day than it is at night. This, because solar dominates in some areas of California, and plugging your EV in during the day when the sun is shining maximizes the amount of renewable-energy generated electricity your EV draws.

On the other hand, other places in the United States lean much more toward wind energy being the primary form of renewable energy generated electricity, for instance, Kansas. There, it can be much greener, typically, to plug in your EV at night, when wind power generation tends to peak.

In the study, Ian Miller, Miller Gençer, and Maryam Arbabzadeh, make these observations in part by calculating the percentage of error in two common EV emission modeling approaches. These approaches ignore hourly variation in the grid and temperature-driven variation in fuel economy, among other variables.

MIT Energy Initiative Research Associate Ian Miller. [Photo Credit: MIT]
Their results establish that the combined error from these standard methods exceeds 10 percent in 30 percent of the cases, and reaches 50 percent in California, which is home to half of the EVs in the United States.

“If you don’t model time of charging, and instead assume charging with annual average power, you can mis-estimate EV emissions,” Arbabzadeh told MIT News. “To be sure, it’s great to get more solar on the grid and more electric vehicles using that grid. No matter when you charge your EV in the U.S., its emissions will be lower than a similar gasoline-powered car; but if EV charging occurs mainly when the sun is down, you won’t get as much benefit when it comes to reducing emissions as you think when using an annual average.”

Seeking to lessen this margin of error, the researchers used hourly grid data from 2018 and 2019 — along with hourly charging, driving, and temperature data — to estimate emissions from EV use in 60 cases across the United States. They then introduced and validated a novel method (with less than 1% margin of error) to accurately estimate EV emissions. They call it the “average day” method.

MIT Energy Initiative Postdoctoral Associate Maryam Arabzadeh. [Photo Credit: MIT]
The researchers demonstrated the usefulness of the average-day method by conducting a case study in the southeastern United States from 2018 to 2032 to examine how renewable growth in this region could impact future EV emissions. Assuming a conservative grid projection from the U.S. Energy Information Administration, the results showed that EV emissions decline only 16% if charging occurs overnight, but more than 50% if charging occurs across midday. According to the researchers, in 2032, compared to a similar hybrid car, EV emissions per mile are 30% lower if charged overnight, and 65% lower if charged midday.

That is a HUGE difference. In sum, the received “wisdom” that it is “always” better for EV owners to plug in at night, when grid use is low(er) and, in some, but NOT all places, wind power generation is higher, is exactly that: Received “wisdom” with LOTS of holes and, in many cases, actually quite problematic and even totally backwards presumptions and assumptions.

MIT Energy Initiative Research Associate Emre Gençer. [Photo Credit: MIT]
The “moral” of the story this particular study tells us: Multiple variables vary considerably across space AND time and if you do not sufficiently take these into account, then the picture you paint vis-a-vis when it is “best” to plug in an EV is, at best, almost inevitably going to be distorted, and, at worst, could be almost completely wrong!

The MIT Energy Initiative researchers did not explore the question of the impact of THE most local form of renewable energy generation — home (rooftop) solar — on the overall “green” EV charging equation. But I am hoping to pull them in for an online Q&A with me/SolarChargedDriving.Com about those types of specific questions. For instance, a key question is the question of whether, if, you live in a high wind energy region but ALSO have home solar, when, if green energy production and consumption are your primary motivation, it is best to plug in your EV.

There are many other questions I have for them:

  1. Is it always greenest to plug in your EV when your home solar system is producing at its peak if you can? If not, when is it best (not to) do so?
  2. At what point/percentage of the electricity going into your EV’s battery from your home solar system — 100%? 75%? 50%? 25%? less? — does the green benefit of your plugging in during solar production significantly diminish, or even flip to being potentially less green than if you did not plug in at that time?
  3. If you already live in a hydro intensive area such as Washington State where about 73% of electricity is produced via renewable means, (how much) is it worth it to add home solar? When should you plug if you live in a high (hydro) renewable energy region if you have home solar?
  4. What are the researchers’ views on solar offset, why do they have these views, and how might a solar offset scenario vary, and why, in terms of the many other variables in play here — local grid mix, time of day, overall draw on the grid around you, etc.?

These are just some of the questions I would love to see this MIT research team try to answer in a Q&A. Do you have any questions you would like to add to the list? If so, I would love to see you post them in the comments stream below and/or Tweet them at me @solarcharge_it on Twitter!

Thank you! Wishing you all many happy Sun Miles®.