In thinking about possible pathways for a low or zero carbon economy, electric cars running on sunshine are an increasingly attractive option.

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

Even though a PV system can be costly, the savings from driving on sunshine versus driving on fossils can easily make up for the initial cost in a few years' time. 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. However, as costs for solar continue to fall and gasoline costs continue to rise, the payback period will become shorter.

Driving 30 miles per day (which is about the average distance driven each day by Americans) requires from 1.9 to 3.2 kilowatts of PV for the four EV models included in the chart below. The Volt is the least efficient of the four models, so it needs almost 50 percent more PV to go 30 miles a day as does the Smart EV. Figure 1 shows the PV system size needed just to run the vehicle, not for any additional home power use.

FIGURE 1: Cost and Performance of 4 Top EV Models

Based on the data shown in Figure 1, we arrive at payback times of seven to twelve years for the PV4EV solution for the four models examined. Somewhat surprisingly, the Tesla has the shortest solar payback because it is highly efficient. I’m also assuming that Tesla drivers are going to drive more each year than would a Smart or Leaf owner (15,000 miles versus 10,000 miles) purely because it’s so much fun to drive a Tesla.

FIGURE 2: PV4EV Payback Times for 4 Top EV Models

After the PV system is paid for in fuel savings, the homeowner essentially receives free power for the remaining life of the system, which is warrantied under California rules to produce at least 80 percent 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, 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 rebates, because those are quickly subsiding for most Californians. Fully 58 percent of solar systems were installed in the first three months of 2014 without the benefit of any state rebates. My cost estimates for solar do, however, include the federal 30 percent Investment Tax Credit, which is available for solar through 2016.

Solar PV costs continue to plummet, so these calculations of payback time and economic viability will continue to become more favorable over time. Figure 3 shows the recent history of solar cost reductions.

FIGURE 3: Reduction in PV System Prices, 2012-2014

Source: GTM Research/SEIA Q1 2014 Solar Market Insight report

The Santa Barbara Community Environmental Council has published a number of engaging case studies about consumers who have opted to drive on sunshine. Based on the numbers shown in the charts above, as well as the environmental benefits of PV4EV, we can expect many more people to start driving on sunshine instead of fossils.

[Update]: Using EPA figures for miles per kWh we get the payback times in Figure 4, 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 4: EV payback times using EPA numbers

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