Electric Vehicle Charging With Solar
Posted on: 2022-09-25In this article I intend to answer the following question:
What is the best time to charge an electric vehicle (EV) when you have solar?
The answer here is for our specific scenario which is:
- We have a 8kW solar system
- We have Net Metering (via Dominion)
- We don't have a Time of Usage (TOU) plan
- We don't have a home battery - we use Dominions grid effectively as a 'battery'
- We are located in Virginia
What is 'best' of course depends on what your goal is. Broadly speaking our order of priorities is probably
Meaning if there are multiple options that are similar in terms of the environment, savings would be taken to choose between them and so forth.
If your scenario and/or priorities differ, your answer may also differ.
Conclusion
Given our scenario:
The best time to charge our EV is off peak/at night.
Analysis
When we installed our solar system we did not go for a system that had a house battery. The main motivation for this choice was that using the grid as a battery is the most environmental choice. When our solar system is producing more power than we need, that power is pushed out onto the grid and will provide power to our neighbors. The main downside in this approach is that when we lose power from Dominion, our solar system doesn't provide energy for the house.
Our solar system doesn't provide enough power for all of our electricity needs, since we bought an electric car. For the purpose of this analysis this doesn't alter the result.
Environment
Any energy we produce via solar is in effect used 'immediately' as we don't have a house battery. The solar electricty might provide power for our house, charge our car, or power our neighbors. There could be an argument that powering our neighbours is less efficient as the energy from our solar panels will need to travel further, and perhaps go through a transformer. I'm going to assume any efficiency loss because of this is negligable in the following analysis, at most a few percent.
The question we are asking here is, how does varying the time we charge our car vary the impact on the environment? That question boils down to what is better for the environment using Dominion as a battery at night or during peak-times? We can simplify in this way because we are making the assumption that any energy we produce will be consumed, and any inefficiencies in where it is consumed are negligable.
In Virginia most power comes from natural gas and nuclear plants. At peak times Dominion may have to use peaking power plants, which are typically less efficient natural gas powered plants.
If we charge during peak times and the sun isn't out or our power requirements are not met by solar:
- We may increase the load on the grid, which may already be stretched (its on peak)
- Might require peaking power plants that are less efficient
If we charge during peak times and the sun is out and meets all our power needs:
- It doesn't really change anything from Dominions point of view, because there is less load from us, but if we didn't charge that surplus would just supply our neighbors.
If we charge during the night
- Power will be from base load plants which will be more efficient stations and probably skew nuclear (which is carbon neutral)
- Being off peak means the grid is not being stretched
This being the case from an environmental point of view it's best to charge at night because
- We can't rely on solar during the day
- If our solar system can reduce the load for Dominion at peak times, that reduces carbon use and load on grid
- If we reduce Dominion load by reducing our load or our neighbors the effect is the same in terms of Dominions power requirement
This argument works largely because in Virigina most power is natural gas/nuclear. If this mix changes - as can be seen in California with Solar - it could lead to Dominion having a surplus of power during the day, and therefore charging in the day might make more sense.
Cost
Only two facts matter here:
- We have Net Metering (via Dominion)
- We don't have a Time of Usage (TOU) plan
This means there is no cost difference when we charge our EV.
Good citizen
This aspect is already addressed on the section on the environment. Charging off peak is the most 'good citizen' thing to do as it reduces the stress on the grid. As more people use EVs the stress on the grid will become increasingly important.
Supporting Information
Peak Demand
Because electricity is the only commodity that is produced at the exact same time that it is consumed (as Peter Kelly Detwiler points out in The Energy Switch), historically, grid operators have been required to keep around certain power plants that are only run during peak times, maybe for less than 40 hours per year. Because they run so infrequently, these power plants tend to be very inefficient and very expensive to operate from both a cost and environmental impact perspective.
Peak demand typically occurs during the heat of the day in summer months, which are the exact same times when solar panels are producing the most electricity. As a result, solar can help offset your pull from the grid during those peak hours, lowering the overall need for power and reducing your individual peak demand.
Dominion
I live in Virginia with Dominion as our electricity supplier. Dominion provide net metering which crudely means we only have to pay for the difference between what we consume from Dominion and what we produce from our solar panels. We do not have time of usage charging (TOU) currently although Dominion does have a pilot program which they call the Off-Peak Plan. It is voluntary and available for customers with a smart meter which we have. Their plan is only applicable for 10k customers though.
Looking at the basic residential rates as 11 and 13 cents for each kWh. The savings don't seem worth it. Even with super off-peak the best case scenario is around 10 cents. That's hardly a significant saving from 11-13 especially when the worst case scenarios are 23 cents, which is nearly twice the price(!).
Dominions basic residential charge does not change based on time.
From the Dominion off-peak plan, the best time to limit Dominion usage would presumably be 3-6 during may to September, or 6-9 and 5 to 8 during the winter. This is assuming that when those times are when the grid is most stressed, and most likely to use less efficient peaking power plants.
Virginia
Virginia's electricity production
| Mwh | % coal | % hydro | % nat gas | % nuclear | % solar | % wind |
|---|---|---|---|---|---|---|
| 8,686 | 4.4 | 1.5 | 56.0 | 29.5 | 6.6 | 0.0 |
Virginias base load seems to be gas/nuclear - which is significantly better than coal.
Power Plants
Combined cycle power plants are approximately 60% efficient.
The thermodynamic efficiency of simple-cycle gas turbine power plants ranges from 20 to 42%, with between 30 and 42% being average for a new plant. source
It is assumed that natural gas power stations are more capable of varying power output, than a nuclear station. This implies the base load in Virginia off peak is likely to skew nuclear.
Other Articles
Elsewhere I have read if your main goal is environmental, the best time to charge your car is at night.
This article discusses the situation in Virginia. It concludes it is best to charge when the sun is out.
Based this articles cheatsheet our best option for Net metering without TOU is
Anytime, but off-peak is best to minimize carbon intensity.
This recent Stanford article claims that it is best to charge an electric vehicle (EV) during the day. I'm pretty skeptical at that result for a variety of reasons. One important difference is that it is for California, which has a vastly different electricity production - 38.0% by solar and 8.4% by wind (!). For that reason alone I doubt Virginia has excess solar energy during the day that is the basis for the papers argument.