Fueling with Sunshine

 

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For many electric vehicle (EV) drivers, it’s only a matter of time before this idea pops up: Why don’t I install PV modules so I can run my car on sunshine?

Our research suggests that approximately one in three EV owners has a home grid-tied PV system. The one-two punch of EV and PV can break the ties between driving and burning fossil fuels—including the coal, natural gas, or other nonrenewable energy sources used by utilities to produce electricity.

PV & EV Work Together

At first, you might think of putting PV modules on top of your car—but due to the efficiency of PV modules and the limited size of a car’s roof, the amount of energy that can be produced isn’t going to help much. Even if you covered the 16 or so square feet of your EV’s roof with PV, on a day of full sun, the energy would cover only a couple miles of driving. The small PV spoiler on the Nissan Leaf is good only for recharging the car’s 12-volt accessory battery. And you’ve probably seen lightweight, low-speed solar concept cars with huge roofs to increase PV surface area.

But a more realistic approach is to use a large-enough grid-tied PV system to offset the daily energy you’re using to charge your EV. When your home PV system is producing power during the day, any appliance that’s running in the house—lighting, television, or an EV charging station—is directly fed by solar power.

Even when you charge at night—as you should, because it’s often cheaper and greener (see “Utility Power Can Be  Greener at Night” sidebar)—your electric car can benefit from clean home-produced electricity. Here’s the reason: the PV system is tied into the grid—pumping excess electrons out to be shared by all utility users. Think of the grid as an energy bank, where you deposit green power when the sun is shining, and withdraw those energy credits when you need them.

Environment & Economics

In many regions of the country, electric utilities rely heavily on fossil fuel as a source. This inspires one viewpoint that EVs are not much greener than a Prius-like hybrid or a small fuel-efficient internal combustion engine-powered car like a Ford Fiesta. Having a grid-tied PV system can put a quick end to this concern.

But even if the environmental question is somewhat settled, debates on the economics of EV and PV are less easily resolved. The traditional argument against both PV systems and electric cars is that you are paying more upfront—on expenses that might take years or decades to pay back through reduced energy and gasoline expenditures.

Arguments for or against these investments are elusive. As usual, the devil is in the details—with a broad set of variables, including:

  • the utility rate in your region (huge factor)
  • how much you drive
  • amenities of the vehicle
  • amount of energy you use at home
  • amount of sunlight that hits your roof
  • PV system cost
  • available incentives (for both the EV and the PV system)

Now add all the uncertainties regarding financing plans for both your car and PV installation—and add in unpredictable and unstable oil and electricity prices. Finally, don’t forget to include the resale value of an electric car. Even if upfront costs are not recovered during the period of ownership—by cheaper fueling and maintenance—any difference is commonly recouped upon sale of the car. Similarly, the value of your home is often increased after adding a PV system. It’s a home amenity (like granite countertops), but one that saves money each month.

For EV drivers, the cost calculation for a PV system is enhanced both by displacing the expense of utility-supplied electricity and the expense of roller-coaster gasoline prices. After you’ve paid off your PV system, the energy generated for your car (and/or home) becomes free.

System Sizing for an EV

For offsetting your EV charging with solar, a system capacity of 2.5 kW is generally a good fit. That’s the size specified by Ford and SunPower for a special bundled deal for a PV system to power a Ford Focus Electric car.

That size assumes that the average output of a 2.5 kW system is about 3,000 kWh per year. If you use a somewhat-generous estimate of 4 miles of driving for every kWh of electricity put in your car, it means about 12,000 miles of driving powered by the sun, each year. This provides a general estimate, so PV systems in sunnier climates can be smaller; cloudier climates will need a larger one. For example, to produce 3,000 kWh per year of solar electricity in Albuquerque, New Mexico, a 1.8 kW system might be all that is required. In Seattle, Washington, you’d need a 3 kW system to produce that same amount of energy.

Of course, you may also want the system to generate green power for your home. You can look at past electricity bills to determine how much energy you are using each month. Depending on your budget, you might decide to have a PV system offset only a portion of your charging or home energy usage. Regardless, knowing how much energy your home and car require is the first step.

Installers & Bids

Unless you are the hardcore DIY type, you’ll need a solar installer to manage your home power project. We recommend reputable, experienced PV installers—rather than box-store solutions—to guide you through every step of the process, including:

  • deciding if solar is right for your site
  • choosing the best hardware
  • finding the best placement on your roof or property
  • managing financing and incentives

Start by compiling a list of prospective installers—word-of-mouth referrals from family and neighbors are helpful, although online reviews can be a starting place, too. The North American Board of Certified Energy Practitioners (NABCEP) offers professional PV installation certification and provides a standard to look to when you’re considering an installer.

System cost is often the bottom line, but most installation estimates should be competitive, so other factors should come to the forefront—such as trust that your installation will operate well for the next 25 years and that the installer will be available to answer all your questions. A good installer will act like a partner throughout the process. Use any proposals to do your own research, and compare the differences between bids in dollars, experience, and technology.

EV Incentives

New EVs with a battery pack of 16 kWh or more qualify the buyer for a federal tax credit of $7,500. EVs with smaller batteries also qualify for a federal tax credit, but on a sliding scale based on battery size. There often are additional incentives available at the state and, sometimes, utility levels. Seek guidance from your installer about incentives, financing, and system leasing versus owning (see “Solar & EV Leasing vs. Owning” sidebar).

Timing It Right

Time-of-use (TOU) electricity rates are an effective tool utilities use to discourage EV charging (and other electricity use) during peak electricity demand hours. TOU rates therefore reward households with net-metered PV systems that produce an excess of energy during the day, and use energy off-peak. That daytime surplus is credited at a higher rate; utility energy drawn at night is purchased at a lower rate.

Here’s an example: In my utility’s (PG&E) service area in the summer, off-peak electricity costs about $0.10 per kWh and peak electricity costs about $0.40 per kWh. If I fully recharge an 80-mile EV like the Nissan Leaf at night, it will cost about $2.40. If I recharged it during the day, I’d pay about $9.60. The point is that you save money on the electric fuel itself by charging at night. On top of that, if my PV system is producing surplus energy during peak hours, I am selling that energy back to the utility at the $0.40 rate­—instead of spending that valuable electricity to recharge my car.

But even without TOU metering, owning an EV and recharging it with a grid-tied PV system can make economic sense, and it doesn’t really matter which comes first—the system or the car. You can put the PV system in now, and be prepared for next year’s EV models to arrive, or start enjoying your EV’s fuel savings now, and install a PV system as the next step.

Web Extras

  • For a list of EV incentives state by state, check out bit.ly/EVIncentives.
  • Check out “Gear” in this issue for a rundown of available EVSEs.

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