I can't say we always charge when the sun is shining, but I try to.
This may be counterproductive, at least on a broad societal scale; but whether it's beneficial to you personally depends on the contract details about how you pay for electricity and how (or if) your solar panels are connected to the grid.
Assuming your solar panels are grid-connected, they provide power to the grid (broadly defined, to include your house) no matter what you do. If you were to completely stop using electricity on a sunny day, then 100% of your production would go to your neighbors and reduce load on the local power station, compared to your not having solar panels. If you turn on every appliance in your home and charge your cars on a sunny day, then most or all of your production will go to that, thus reducing load on the local power station, compared to your not having solar panels. That is, your solar panels provide a benefit no matter what you do. Flipped around, if you charge your car when the sun is up, you'll be charging (partly or completely) from solar power, but that power will
not be going to your neighbors, so the load on the local power station will increase, even though you've got solar panels on your house and the electricity they produce is going into your batteries.
In many areas, grid load rises throughout the day, peaking in the mid-to-late afternoon. At this time, utilities often bring their most expensive and most polluting power stations on-line. Because charging your car, even with a grid-connected solar array on your house, increases grid demand, this results in an increase in pollution from those most-polluting power stations.
Of course, there are a lot of caveats and exceptions to this rule, most of which are regional or site-specific:
- Is your house grid-connected? If not, you'd want to charge when your panels are producing the most electricity; the grid doesn't matter.
- What's the overall power production look like for your area? In particular, what types of peaker plants exist in your area, to provide that extra late-afternoon "kick" in production? A few areas in the US, such as parts of California, now have so much solar power installed that they over-produce around mid-day, and have to throw electricity away as a result, so charging at mid-day might be beneficial.
- What's the overall demand curve in your area, and on the day you're considering? This varies regionally, seasonally, and on a day-to-day basis.
- Do you have a battery system, like a Tesla Powerwall, to store solar power? If so, then your house's electricity demand is likely to be low throughout the day, and you should probably charge your car whenever you're producing the most power, although details will depend on your system.
- Are you on a flat-rate or time-of-use (ToU) billing system? This doesn't affect the environmental impacts of when you should charge, but it's typically designed to motivate electricity use to match the utility's costs, and indirectly the environmental impacts, so charging when rates are low is a decent proxy for when charging imposes the lowest environmental costs.
- What are the details of your solar power installation's grid connection? That is, are you on net metering, a feed-in tarriff, or something else? Is billing done monthly, or is there a time-of-day component? These will affect the economics of it, but likely not the environmental costs of charging.
You may want to consult with local resources to figure out when the best time to charge is, whether or not you have solar panels on your house. Ask your local utility, public utility commission, or local advocacy groups. For me, the grid is managed by an industry group called ISO New England, and
their Web site has a real-time system demand graph that anybody can monitor. I buy my power through a "GreenUp" program and the
Green Energy Consumers Alliance, which has a program called
Shave the Peak, in which they send out alerts when consumption starts to peak on summer days. I then know not to charge my car at those times (and to otherwise minimize my electricity consumption). I've heard that some utilities (mostly in California) have programs that can communicate with some EVSEs, like some JuiceBox units, to have the EVSE initiate charging when the grid has a surplus of "green" energy. (See the
JuiceNet Green page for details; however, timed charging doesn't work all that well with JuiceBox EVSEs and Teslas, so from a practical perspective this isn't the greatest option for Teslas.) I mention all of this simply as examples of resources you might be able to use. Unfortunately, these programs, resources, and grid details vary a lot regionally, so you'll need to research both the issues and resources in your own area.