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Why Use Rooftop Solar Panels to Power Your Electric Vehicle?

Chip Gaul's picture
Residential Solar 101

Chip Gaul is a researcher and blogger on Residential Solar 101. He focuses primarily on solar PV markets, trends, prices, and additional incentives. He graduated with honors from the University...

  • Member since 2018
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  • Jun 28, 2012

Solar cells stand to place an increasingly prominent role in augmenting the practicality of electric cars.  In theory, the idea of using solar panels to charge electric cars seems a perfect combination. The sun is the planet’s greatest source of free energy and anyone with the capacity to harness it is at an advantage. The reality is of course more complicated:
Since most people are out during the day, the car will be away from the domestic panels used to charge them. At night, when the car will be parked unused, there is no sunlight with which to charge the car. However, this is a straightforward problem to work around. Instead of the solar panels linking directly to the car, they can instead be used to charge a battery bank when the car is in use. When the car is parked at home, the battery can then be used to recharge the car. SolarCity, one of the largest solar leasing companies, has recently begun offering such battery banks as a promotion to their customers –  ostensibly as a back up source of electricity in the event of a power outage – but likely also spying a gap in the market to be a provider of such equipment.
An alternative is that the panels are instead used to feed the electricity grid during the day, then when the car is plugged in at night the power is taken straight back from the grid. This often has an added monetary advantage to it, since through net-metering, electricity is typically charged more cheaply at night.  Then from a financial standpoint, you’re effectively getting out more than you put in.  Don’t plan on making big bucks on such an arrangement though – most utilities in the US will only allow you to generate credits to your bill instead of out-and-out revenue from net-metering.
Cities Play Catch Up

Despite the future of automotive efficiency lying very firmly in the solar panel/electric car combo, if everyone suddenly adopted the practice the situation would quickly become untenable. The electrical infrastructure of cities doesn’t currently possess the capacity to support an entire block’s worth of electric cars all charging at the same time, as charging an electric car can consume up to three times as much power as an average household.
For this reason it may be worth  considering making the investment before your neighbors do. If you wait too long, there could well be a fight over who has the right to utilize what power the local grid is able to muster. It will likely be some time before the necessary upgrades are made that will allow for the increased current that the vehicles will demand. The catch-22 is that upgrades are unlikely to be initiated until there has been enough uptake of electric vehicles to demand such improvements.
Another issue is if you park on the curb rather than in a garage, you’ll be a fair distance from where you can actually plug your car in to charge. Unless you want to trail a few dozen feet of power cables down the sidewalk, it will be difficult to maneuver your vehicle into a position where it can be charged.
However, things are slowly changing for the better. Many cities are introducing specialized parking spaces reserved for electric cars that also provide a socket to charge your vehicle while it’s parked. In the San Francisco Bay Area, which is home to the three largest suppliers of solar panel installations, there are areas with very high concentrations of such spaces, making electric cars not only viable, but also practical.
Looking to the Future

The attraction of electric cars is easy to see. With the world’s finances still in freefall and the price of gas increasing almost exponentially, the appeal of being able to power a car through renewable means becomes ever more appealing. As well as the financial payoff, the environmental benefit of electric vehicles is obvious and is only one of a number of reasons why you should seriously consider investing in what will likely become the vehicles of the future.

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Paul  Scott's picture
Paul Scott on Jun 29, 2012

I’m happy to see an article that showcases EV/PV. There were a couple of items that need clarification, however.

While you can install a battery back up for your solar system and charge it during the day so you can use that energy later, it’s more cost effective to use the grid as your “battery”. In other words, sell your excess energy to the utility for credits, and then use those credits at night when you charge your EV. This saves the cost of the big battery back up system for now. At some point in the future when the battery packs from the initial wave of EVs are replaced, these used packs will be cost effective for use in the home.

The net metering you talked about was pioneered here in CA back in the 80’s to encourage solar PV installations. It works great. What you were referring to is actually “time of use” (TOU) rates where there is a differential in the price of energy used during on-peak hours vs., energy used during off-peak hours. When I installed my 3 kW PV system in 2002, we stayed on the normal rate plan. Our electricity bill dropped a lot, but we still had a small bill. When we changed to a TOU rate plan, we went negative with the utility. For those who use very little energy during peak hours, this is a huge boon. We “sell” our excess energy to the utility for about 30 cents/kWh, and then “buy” it back at night for about 10 cents/kWh. This dropped our electricity bill to about $100 per year for both our home and EV. We calculated that our PV system paid for itself after the first 8 years of use, but it’ll keep generating clean, renewable energy for decades to come.

Lastly, most progressive utilities, SoCal Edison top among them, are watching closely where people who buy EVs live. When a particular neighborhood gets two or more EVs, the utility sends a crew out to swap the transformer for a new, more robust model that can handle the additional load during off peak times. This will be a natural progression of replacement that is already covered in the rates, so no increases will be needed to cover it.

Pam Kelly's picture
Pam Kelly on Jul 3, 2012

You may want to read the excellent analysis of US power use by the former energy planner from the Tennessee Valley Authority.  He was hired to see whether it lwas even possible to anticipate a ZERO Nuclear power and ZERO fossil fuel energy use future.  (He said he was very sceptical to start with…)  Among many solutions—only ones that have been CURRENTLY developed, reday for mass use,  he came up with a solar PV to car scenario, where people charged thier cars at their place of employment, during the day, (with sheets of PV covering parking lots, a practice already in place in some locations) and the battery power of millions of transportation vehicles becomes the nations’ commuter and overnight electrical storage system.  Ingenious.  The book is: Carbon-Free and Nuclear-Free:  A Roadmap for U.S. Energy Policy by Arjun Makhijani.  

IEER Press, Takoma,  MD 2007

He is also a very articulate and humorous speaker  Pam kelly

Frances Smith's picture
Frances Smith on Sep 22, 2012

This is probably a good idea as one of the problems of having electric cars aside from the availability and price of its auto parts is that it takes a long time to charge. I hope the solar panels will be the answer that electric car owners are looking for. It's interesting and it seems possible. Let's just wait and see what will happen in the future. 

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