Why it makes sense to pair solar with electric vehicles

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Electric Vehicles running on sunshine are an increasingly attractive option in a low carbon economy.

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Greentech Media

Ford_Solar_EV_310_218In 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. have solar panels 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 wereinstalled 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.

Source: Greentech Media. Reproduced with permission.

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  1. Askgerbil Now 5 years ago

    Electric vehicles and solar PV systems make for a good technology pair.
    More electric vehicles mean lower costs and faster development of batteries for energy storage.
    The electric vehicles energy storage makes a good backup energy store in extreme peak demand periods.

    A “half-way” technology that may encourage some people thinking about buying a solar PV but are waiting for energy storage and electric vehicle costs to fall more –
    a. Solar energy produced when home owners are at work can be used to produce hydrogen and oxygen.
    b. Petrol and diesel engines run at higher efficiency and on leaner fuel mixtures when small amounts of hydrogen and oxygen are injected into engine cylinders along with the fuel.
    c. Saving petrol and money for an existing vehicle is a good use of solar energy until making the upgrade to an electric vehicle.

  2. Malcolm Scott 5 years ago

    Pairing EVs with solar does make sense, but there can be a downside if not monitored. Instead it might be better to just use renewables at night from the grid.

    Hard to disagree with the payback time being the same as for any behind the meter self- consumption of solar PV. However, with powering an EV using solar, it does warrant a larger system which will have a lower installed cost/kW. This will provide a better solar pv payback time for the background household demand that the house owner would otherwise have purchased – all goodness.

    As a Volt owner I take exception to Tam Hunt’s claim that ‘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’. This is absolutely incorrect.

    I don’t disagree with the broad thrust of the article assuming some favourable
    parameters, BUT…. Tam has made an error by using the wrong method for analysis. This produces a big error with the Volt and a small error with all the other models. Tam has misused battery capacity. He’s used battery size rather than the capacity that the OEM makes available for the design parameters it chooses.

    There is a raging thread on this at GreenTech Media, and for some reason Tam is not getting it. A better indicator is US EPA data for kWh/100 mi. If my memory serves me correctly the EPA measures at the wall and thus includes charging inefficiencies. Tesla Model S 60kw and Volt come in the same at 2.9 miles/kWh, Leaf 3.3, and the new efficiency king, if not range, the BMW I3 BEV at 3.7 (using the combined cycle, which is rather conservative and easily exceeded by EV drivers in all but cold days with everything on).

    As a Volt owner with solar PV, I’d also call into question the analysis where the EV charge rate is higher than that provided by solar PV (less background household demand) during the day. EV charge rates come in at approximately 2.4, 3.3, 6.6-7.2, 10, & 20 kW as determined by the vehicle, and in at least one case down selectable
    depending on the EVSE. It will take a very large solar PV system to provide excess household power that can charge at these levels throughout the charging window such that there is no import from the grid. In the real world you tend to use the grid to supplement the charging if using solar pv. If there is not enough solar irradiance, you might be using more from the grid than you like, and then you might as well use the night time tariff, and green if you prefer.

    The Volt also has a low 6 amp selectable charge rate that does help prevent
    using grid power if charging during the day and no import of power from the
    grid is desired (and then you might run out of sunshine hours instead).

    My experience is that in practice it’s much harder to manage powering EVs from
    home solar than you might first think. There are so many variables to consider, and some of these change throughout the day. And in any case the EV needs to
    be at the site of the solar PV during the day, which is often not the

    • David Osmond 5 years ago

      Thanks for your comments Malcolm. I was surprised that the Volt appeared to be so much less efficient, so thanks for correcting this. It is mind boggling that Tam Hunt does not understand his mistake, and won’t correct his article.

      Thanks also for your comments on charging rates rates etc. too.

      Another benefit of using EVs if you have decided to go off-grid is that if you have a very bad week of solar resource during the middle of winter, and solar production is down and battery levels are low, then you can drive to a charge point and top-up the batteries.

    • Rockne O'Bannon 5 years ago

      Tam Hunt does not get the Volt argument because he is biased, plain and simple. No mystery there. Everything is Tesla with this guy. He is flattering the Volt by attacking it.

      If you confront him with it, he will admit it. Even he acknowledges that his articles need disclaimers. He blames the editors for not putting them on his articles.

  3. Chris Sanderson 5 years ago

    Yes, you’re right. It’s a great pairing. There’s another factor for rural owners – energy security, especially when there’s no public transport and you live too far to walk to the shops.
    If the oil industry insiders are right, we will have an ‘Oil Crunch’ in the 2015-17 timeframe as world demand exceeds world production levels. Add the research paper on the NRMA website and you discover that Australia only has 3 weeks oil supply vs the recommended 9 weeks…../Chris

    • Farmer Dave 5 years ago

      I agree with all your points Chris. I live about 8 km from the nearest shop and buying an electric vehicle (or converting a vehicle) is definitely on the agenda for us. As well as being concerned about the petroleum supply situation, I am not at comfortable about my family’s greenhouse gas emissions, which are almost totally due to vehicle use. For potential early adopters of EVs it’s a tricky timing question about when to adopt the new technology, particularly when the price tag is pretty steep.

      Thanks for the article, Tam and Giles – some interesting data in it. In my case, due to some unplanned circumstances, I have ended up with an oversized PV installation. That has lead me to think it would be great if a line of EV chargers were available that could take the DC output from a PV array directly, rather than converting the DC from the array to AC in the inverter and then converting it back to DC in the charger. Clearly there would be technical issues to resolve, such as whether a maximum power point tracker would be used. However, in the case of my oversized installation, it would mean extracting more solar power under good conditions.

  4. Rockne O'Bannon 5 years ago

    No disclaimer here, Tam Hunt?
    Tam Hunt is a lawyer who writes a lot of articles on electric vehicles, chiefly the Tesla. He trashes other vehicles, just as he does here, cites questionable research, and produces “data driven” articles for green sites. He often cites data for Santa Barbara. Anyone want to guess why? Should we wonder why he is always boosting Tesla? Of course, especially since the other cars he mentions are more popular.

    This is PR, not journalism. He says it is not straight journalism. I call it bent journalism.

    The last time I ran into him, he claimed that his editors failed to show a proper disclaimer for him. Oooops. They did it again.

    Regarding the article, for anyone who receives a FIT, these data will not apply. For anyone who gets low off peak rates, they would be better off just charging their cars with coal generated electricity off peak than buying a solar array just to charge their vehicles.

    Don’t let Tam Hunt ruin a good site.

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