Most EV charging happens at night, when cars are sitting idle, and there are many benefits to that. Drivers wake up to a “full tank” and most utility companies offer cheaper time of use energy plans to incentivize usage when demand is lowest. This has worked well so far: EV drivers save money charging and there is little strain on the grid when electricity is needed for time sensitive appliances like air conditioning or stoves.
But things are changing. Some states – notably California – are rapidly increasing their renewable energy production, at times generating more clean energy than can be used.
Are EV drivers taking advantage of this plentiful, clean energy to charge their cars?
New analysis of over 29,000 charging sessions suggests that EV charging isn’t happening at the right time of day to maximize the use of solar energy. This chart looks at California, the leading US market for both EVs and solar generation.
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Key Findings
- The optimal time for solar energy production is between 10am and 4pm, when the sun is highest in the sky.
- Only one in four California charging sessions (25%) occurred during this window.
- The remaining 75% of charging sessions occurred at night, when the grid is likely being powered by more carbon-intensive sources - but when residential energy prices are often lower.
- But over 40% of fast charging sessions occur between 10am to 4pm.
- Although fast charging only makes up around 2% of total charging sessions, the energy demand from fast charging is nearly equal to overnight charging.
While EVs that are charged with electricity from fossil fuel-fired power generation are still more efficient than gasoline-fueled vehicles, EVs charged by renewable energy can operate with nearly zero carbon emissions. As the grid relies more on renewable energy, EVs will become a better and greener choice, so long as we are able to use that clean energy for our cars.
Note that wind power, another renewable energy source, is often plentiful at night. States like Texas get a peak in wind power energy production at night.
Lessons for EV Charging
Two years ago, we wrote an article that looked at California’s growth in EVs and solar generation, noting that renewable supply has been growing faster than EV charging demand, and that given those trends, the US power grid will be able to handle the transition to electric vehicles. And, although EV charging is not a strain on the grid today, eventually vehicles will have to charge when renewable energy is being generated.
Here are several key lessons from our study.
Lesson 1: The cleanest time to charge varies by state
California is a leading market for both EVs and solar generation, and that means that its grid operates differently than across the rest of the country.
In general, the carbon intensity of the electric grid is lowest between midnight and 6am, which means it is “cleaner” to generate additional electricity at night, when most drivers are already charging.
On the other hand, in California, the carbon intensity of the grid is lower midday than at night. The same is also true in parts of New Mexico, Wyoming, Colorado, Arizona, and Utah.
Lesson 2: In high solar regions, such as California, there need to be more Level 2 charging stations where cars naturally “dwell” during the day
Fast charging already tends to occur in the middle of the day when solar power is prevalent on the grid. Large employers, government agencies, and universities should be incentivized to provide banks of level 2 chargers at their offices and factories, since EVs will normally be parked there anyway unused during peak solar production. This will naturally shift charging to take place when solar is active.
Lesson 3: Utilities need to make it more expensive to charge in the evening, which is the dirtiest time to use electricity
Most Level 1 and Level 2 charging happens overnight, but outside of California, a lot of home charging starts to ramp up in the late afternoon and evening when people get home from work. Across most grids, evening is the most carbon intensive time to charge your EV. Utility plans can make it more expensive to start charging in the evening with special EV charging rates and time of use plans. This would shift charging time to after midnight, when the carbon intensity and grid demand is lower.
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Lesson 4: Time of use plans work
In California, we see a surge in charging sessions (40x more) begin between 12 and 12:05 am (482 sessions) as opposed to those that began between 12:05 and12:10am (12 sessions). This pattern was only seen between midnight and 12:05, likely due to time of use utility plans that give discounts on electricity when demand is lowest – usually starting at midnight.
We can also see a charging peak at 4am - 5 am, likely due to people who set their cars to be “topped off” at the time they leave for work, when energy is still inexpensive (and pretty clean).
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Outside of California, we do not see the same surge in Level 1 and 2 charging sessions at midnight. The prevalence of evening charging and lack of an after-midnight surge is due to fewer dedicated time of use plans for EV drivers across the rest of the country, or less education about plans that already exist.
We recommend actively encouraging midday EV charging in solar-friendly regions and middle-of-night EV charging in wind-friendly regions. For instance, Bear Valley Electric Service (BVES) and Marin Clean Energy (MCE) in California offer off-peak rates for daytime charging that overlaps with solar generation. In part of Texas, EV drivers get night time rates as low as $0.00. It's a bonus when this free electricity comes from clean wind power.
When implemented and communicated well, these time of use programs work. The MCE time of use incentives shifted 72% of charging sessions to the off-peak window from midnight to 3pm.
Lesson 5: Everyone should use software solutions that enable green charging
Many utilities offer managed charging programs that can automatically start and stop EV charging when the kWh price is low, based on real-time demand data. Utilities should also provide access to data, software, and integrations that allow drivers to charge preferentially off clean energy.
A collaboration between WattTime and EnelX does just this for JuiceNet chargers that add on an additional software package. WeaveGrid and Wallbox partnered for a program in Colorado to time EV charging with production of wind energy. Similarly, the partnership between MCE and ev.energy created software that allowed the 30% of homeowners with solar setups to automatically shift charging to when their arrays are active. Between solar charging and time of use incentives, MCE successfully avoided more than 317 metric tons of carbon from EV charging in about three years.
Widespread promotion and adoption of this sort of software should be incentivized and prioritized since it takes into account the unique grid makeup in every region.
About our data
Charging session data was collected from a random sample of anonymized Tesla owners using Recurrent’s free owner monitoring platform over the course of a seven day period (June 7 - 13, 2024).
- Total number of vehicles taken data from: 8797
- Number of vehicles from California: 1999
- Total unique charging sessions: 29021
- Number of California sessions: 5931
- Average duration for charge sessions overall: 201 minutes
- Average duration for fast charges: 33 minutes
- Average duration for level 1 or 2 charges: 216 minutes
- Average KW for fast charges: Assumed to be 100 KW (we didn’t have exact data given variances in charge speeds during fast charging events)
- Average KW for level 1 or 2 charges: 4.35 KW
Data analysis was done by Cyrus Lerner, a senior in high school from Seattle, Washington. He is extremely passionate about coding and math, and outside of academics enjoys lifting weights and playing soccer. He is particularly interested in EV research, and hopes to do more work in the field in the future!
If you have comments or questions on our methodology, or want to connect directly for any reason on this subject, let us know at contact@recurrentauto.com.
