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  • Writer's pictureTamlyn Hunt

Driving (finally) on sunshine

The PV4EV solution

Not many feelings can rival driving on sunshine – driving an electric vehicle powered by solar panels on your home. I finally achieved this state of transportation Shangri La at my home in Hawaii. I had solar panels on my last house but hadn't yet made the leap in Hawaii to an electric car. In California, where I lived previously, I had an electric (my little Fiat 500 EV) but no solar on my house.

I have now finally married these two technological miracles and I can say truthfully that almost every mile I drive is powered by the rays of sun that fall on my roof.

A recent survey found that 32% of electric vehicle (EV) owners in the western US have solar panels (PV) on their homes. We don’t know how much of the miles driven come from those panels because they may not be sized appropriately to supply the home demand and the vehicle demand. Regardless, it is clear that more and more homeowners will, as prices for PV and EVs continue to fall, opt for what we can call the PV4EV solution.

Even though a PV system can cost a pretty penny the savings from driving on sunshine versus driving on fossils can make up for the initial cost in a decade or less. In crunching the numbers for various models I found that payback times for the PV4EV solution are generally around ten years—good but not great, at least from a purely economic perspective. As costs for solar continue to fall and gasoline costs continue to rise, this payback will become shorter and shorter.

To drive 30 miles a day (about the average distance driven each day by Americans) requires from 1.9 to 3.2 kilowatts of PV for the four models included below. Figure 1 shows the PV system size needed just to run the vehicle, not for any additional home power use.

Figure 1. Info on four EVs available today.

Using this data from Figure 1 we get 7-12 years payback times for the PV4EV solution for the four models examined (Figure 2). Perhaps surprisingly, the Tesla has the shortest solar payback because it is highly efficient and I’m assuming that Tesla drivers are going to drive more each year than a Smart or Leaf owner (15,000 miles versus 10,000 miles)—just because it’s so much fun to drive a Tesla.

Figure 2. Payback times for solar PV purchase from fuel savings from four EVs available today.

[July 21 addition]: Using EPA figures for miles per kWh we get the payback times in Figure 3, ranging from 8-12 years, with the Volt faring better in comparison to the EVs, apparently because the Volt is programmed to be more cautious in using its battery capacity than the EVs.

Figure 3. Payback times using EPA figures for m/kWh.

After the PV system is paid for in fuel savings the homeowner enjoys literally free power for the remaining life of the system, which is warrantied under California rules to produce at least 80% of full capacity after 25 years. That’s a good deal in my book.

I haven’t considered any cost premium for the EV purchase in my PV4EV calculations because each car is essentially a high-performance vehicle, when compared to its fossil-powered equivalents, so the cost is justified by the value. This is particularly the case when we include, as I have done, available rebates and tax credits. There is a societal cost, of course, to these subsidies, but the cost is minuscule compared to the potential benefits of accelerating the transition away from fossil fuels.

These subsidies won’t stick around forever and nor should they (both have phaseouts already built in). However, it seems very likely, given recent price reduction trends, that these EVs will continue to represent a good value proposition even as rebates and tax credits fade away – particularly when we consider the cost savings from driving on sunshine versus fossils.

My cost estimates for PV don’t include any state rebate, because that is going away quickly for most Californians. Fully 58% of solar systems were installed in the first three months of 2014 without any state rebate. My cost estimates for solar do, however, include the federal 30% tax credit available for solar through 2016.

Solar PV costs continue to plummet, so my calculations will continue to become more favorable over time. See Figure 3 for the recent history of solar cost reductions.

Figure 3. US solar cost reductions (source: SEIA market insight report Q1 2014).

The Santa Barbara Community Environmental Council has a number of engaging case studies of people choosing to drive on sunshine. Based on the numbers above, as well as the obvious environmental benefits of PV4EV, we can expect many more people to start driving on sunshine instead of fossils.

Tam Hunt, J.D., is owner of Community Renewable Solutions LLC, a renewable energy project development and policy advocacy firm based in Santa Barbara, California and Hilo, Hawaii.

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