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How Many Solar Panels Does It Take to Power an Electric Car?

It's not rocket science, but estimating the amount of energy requires some calculations

Calculating the number of solar panels needed to power an electric vehicle is fairly simple. Photo courtesy of the Zero Energy Project.

My wife and I built a zero-energy home in 2015. The energy model we conducted as part of that process said that we would need to generate about 6200kwh per year to provide the energy our home was projected to use. So, we installed a solar electric system with that in mind. As it turned out, due to our well-planned energy-saving measures and conservation practices, we actually produced more energy than we used.

Our utility interconnection (net-metering) agreement allows us to receive a credit for each kwh that we generate in excess of our monthly usage. We can carry this credit forward each month, to cover the cost of the energy we use in the winter when our production is lower. But every April any remaining credit is lost to us and we start over at zero on our energy-bill balance sheet. As utility policies go, that’s not too bad. We essentially gain the retail value of our excess monthly generation and have the opportunity to build a credit over the summer to use the following winter.

Since we were producing more energy than we used, we were leaving a lot of electricity on the table. We used only 60% of the energy we generated in 2016 and 80% in 2018. The retail value of that electricity is about 10¢ per kwh, so we were donating a couple hundred dollars per year to the grid. Then we realized that we could capture the value of our excess energy production by using it for transportation instead.

So at the end of 2017, we purchased a used Nissan Leaf. It was two years old and had just 7000 miles on the odometer. We have now owned it for almost two years and use it exclusively in a 30- to 50-mile radius. We always charge at home and pay nothing for fuel, and as with all electric vehicles our maintenance costs are extremely low. Actually, so far, our maintenance cost has been zero. So our solar panels power both our home and all our local transportation while significantly lowering our cost of ownership for both home and car.

Solar-powered transportation

By installing sufficient solar collectors to power both your home and an electric vehicle, almost anyone planning a zero-energy home can reach the energy potential of a positive-energy home. While my household backed into the positive energy zone after our home was completed, many people want to know how they can plan ahead to power their home and car from their solar panels. Most energy modeling will provide fairly accurate guidance as to the number of panels needed to fully power a net-zero-energy home. So the critical question becomes, how many additional solar panels does it take to power an electric car?

It’s not rocket science, but estimating the amount of energy requires some calculations.

Miles you drive

If you’re a daily commuter, this should be pretty easy to estimate. Most early electric vehicles, like our Nissan Leaf, have a range of about 90 miles per charge. These are “around town” cars, and many people would probably drive them between 5000 and 10,000 miles per year. This is rapidly changing with the availability of longer-range EVs, such as the 238-mile-range Chevy Bolt, already on the market; a variety of new longer-range EVs coming in 2020; and many more coming on the market between 2020 and 2025. Five long-range full-electric pickup trucks and 13 electric SUVs will also soon be available.

Efficiency of your vehicle

Currently, most EVs are small to midsize sedans, so the efficiency among them is fairly similar. Our 2015 Leaf travels 4.3 miles on 1kwh. My friend’s solar-powered Tesla Model 3 gets 4.4, and his Chevy Bolt is rated at 3.5 miles per kwh. Of course, that varies with your driving style, the terrain, and season, as well as with air conditioner and heater use.

During cold winters or hot summers, the mileage may drop by up to 25% due to temperature extremes. On the other hand, during the moderate shoulder seasons and in mild climates, most EVs get considerably better miles per kwh. For example, the Chevy Bolt can drive up to 6 miles on 1kwh during mild weather. Incidentally, fuel economy in fossil-fuel cars also drops significantly in extreme weather, but few people seem to notice.

Annual output of one solar panel in your climate

The easiest way to learn how much electricity one solar panel will produce annually is to ask a local solar contractor. If you already own a solar electric system, you probably have access to the actual production data through a website, user manual, or mobile app. You can also look up typical system-performance information for your area through PVwatts.org.

Once you know how much electricity a single panel will produce each year, the next step is to divide the number of miles you intend to drive per year by the miles per kwh of the vehicle. That gives you the kwh needed for driving. Then simply divide your kwh needed for transportation by the expected yearly output of your solar panels.

Here’s how it works out for us. In the first two years of EV ownership, we have averaged 4000 miles per year. Our Nissan Leaf gets an average of 4.3 miles per kwh. Each of the solar panels we installed generates 360kwh annually. Here is the calculation:

4000 miles per year ÷ 4.3kwh/mile = 930kwh needed to power our vehicle per year

930kwh ÷ 360kwh/panel = 2.5 panels needed to provide 930kwh per year

Obviously, you would want to round up to the next full panel in this situation. The little bit of extra output will give some room to increase your solar-powered driving distance as well as accommodate an acceptable margin of error. Also, solar output will decline by about 0.8% each year as panels age and car batteries will lose a bit of efficiency over time, just as internal combustion vehicles decline in efficiency as they age.

What if you don’t have solar panels for your electric vehicle? It’s still more affordable and less polluting to drive an EV than a fossil-fuel burner, even if you purchase electricity from the grid. To compare for yourself, see the U.S. DOE’s cost comparison tool. So don’t let that stop you from transitioning away from internal combustion vehicles. But if you’re planning to install solar panels, consider adding sufficient panels for your daily transportation needs, and take a giant step toward a more economical zero-energy, zero-carbon life.

This post originally appeared at The Zero Energy Project.

14 Comments

  1. AdamHoff | | #1

    I'm not sure if it's factored into the official kWh/100 number or not, but regenerative braking makes a huge difference as well. A friend of mine owns a Leaf as well, and although he has a 14 mile round trip commute he only uses around 10 miles of range. Being able to recapture the car's kinetic energy instead of dissipating it as heat gives him a free trip to work about once a week. Obviously this heavily depends on the route being driven, but the emergence is interesting.

  2. User avater
    Jon R | | #2

    > zero energy, zero carbon life.

    While it probably is a step in that direction, zero net annual energy does not mean zero carbon. If you are concerned about carbon (and you should be), start calculating it.

    Even more so than gasoline powered cars, low speeds boost EV efficiency.

    1. Vivian Girard | | #3

      I agree with Jon R; while currently trendy, the environmental benefits of zero-energy and electric vehicles may be vastly overrated. This reminds me of the very much over-hyped promise of biofuels in the early 2000's (1). I'll stay on the topic of electric cars this time. My wife says that I should change my last name to "Skeptic"; she is probably right...

      A recent (Jan 2019) academic study from Germany looking at the total amount of energy required to manufacture Lithium Ion battery (LIB) reveals some surprising numbers (2).  It shows that manufacturing 1 Wh worth of battery requires an outstanding 1153 Wh of primary energy. Considering that Worldwide so much is being invested in that technology, that’s an enormous environmental burden.

      As I understand it, the main point of the paper is that most recent studies looking at the embodied energy of manufacturing LIB  have only focused on the (already very energy intensive) manufacturing process. These studies have typically overlooked the even more energy intensive extraction and processing of the raw materials involved in the fabrication of LIB.

      To put it into perspective, a small electric car like the Nissan Leaf comes with a battery of 40 kWh. If this paper has it right, it takes some 45,000 kWh of primary energy to make such a battery.  At the high end, the Tesla Model X SUV is available with a 100-kWh battery, and this requires a staggering 115,000 kWh just to make the battery pack.  
      A household with two average electric cars with 12 years of useful life would require some 130 kWh “worth of batteries” with an embodied energy of about 150,000 kWh.  

      A typical US home uses 12,000 kWh/year -compared to 3,000 kWh in for a German home(3).  Therefore, over a twelve-year period, a US household would consume about as much energy (150,000 kWh) by owning two electric vehicle's as they do to power their low efficiency home (144,000 kWh).   For a German household with the same two electric vehicles, that ratio is four to one. The energy required to manufacture the rest of the vehicle, powering it, and build and maintain the infrastructure associated with the road network etc. is not even included in that energy budget.  

      I don’t have the proper knowledge or background to verify the details of the paper mentioned above, but it looks like bona fide and non-partisan academic research from one of the highest ranked German engineering university.  And of course, the ordinary gas powered car (or SUV for a majority of Americans nowadays) is also a supremely inefficient and environmentally devastating  to move one person around on a daily basis. But assuming that this study's conclusions are within the right order of magnitude, it provides some sobering figures. Lithium Ion batteries hardly look like a panacea when it comes to energy storage and transportation. 

      This is not breaking news. Among others, ADEME (the French equivalent of the US Energy Information Administration -EIA) had already come to a similar conclusion in 2016 (4): “A report by the French Agency for Environment and Energy Management (Ademe) in 2016 highlights their lower greenhouse gas emissions and reduced dependence on fossil fuels; but it doubts that electric vehicles can provide ‘a real solution to energy efficiency issues’ and concludes that the ‘negative impact on the environment, mainly during the manufacturing phase, [is] comparable for electric vehicles and internal combustion engine vehicles’”

      Another troubling finding according to that same article: "Energy research firm Bernstein calculates that the number of cars on the world’s roads will double by 2040. Unless electricity is taxed as heavily as oil, the modest cost of charging an electric vehicle could encourage greater consumption. And the belief that electric cars are clean could lead to even greater urban congestion." 

      (1) https://www.nytimes.com/2018/11/20/magazine/palm-oil-borneo-climate-catastrophe.html?searchResultPosition=5
      (2) https://www.sciencedirect.com/science/article/pii/S2212827119301015
      (3) https://wec-indicators.enerdata.net/household-electricity-use.html
      (4) https://mondediplo.com/2018/09/10electric-cars-rebound

      1. Eric Habegger | | #4

        I think you are overthinking this. I scanned the articles you reference without going in depth and it seems to me the conclusions you reached are very extreme. For instance, the modediplo article isn't credible. It states that since EVs are much heavier than ICE cars they are less efficient. That ignores that EVs can regain energy from braking which ICE cars cannot. There are numerous other contextual comparisons throughout the articles that just don't hold up.

        One of the biggest is that there is never an apples to apples comparison in any of the articles between the embodied energy required to make ICE cars versus EV. No comparison is valid without explicitly doing that.

        My understanding, though incomplete, is that if Tesla implements the Faraday technology they acquired when they bought the company it will result in a substantial reduction in energy required to make LIBs. That is because they will no longer require a heat intensive drying of the solvents currently used in making batteries. Instead it will involve a new process where the ingredients are directly deposited without using a solvent. But even apart from that I just think it may be poor critical thinking to assume from those articles that LIBs over the full lifecycle don't compare favorably to ICE engines. It doesn't pass the smell test.

        1. User avater Expert Member
          Dana Dorsett | | #5

          >"I just think it may be poor critical thinking to assume from those articles that LIBs over the full lifecycle don't compare favorably to ICE engines. It doesn't pass the smell test."

          That's right, especially since that "full lifecycle" is likely to exceed it's use as an EV battery, with another large decade of useful life as a grid storage or home battery, once the critical quantities of used EV batteries develops. In Europe Nissan is already packaging up and selling repurposed used Leaf batteries into the distribution-grid support market. A similar approach is likely to be coming to a grid near you before 2025:

          https://www.tdworld.com/electrification/article/20972759/the-power-of-reusing-electric-vehicle-batteries

          How much of the ICE drivetrain can be repurposed at a profit (other than as scrap metal) to extend it's lifecycle?

          It's not apples to apples.

          But it's also silly to belief that Lithium ion technology is guaranteed to win the EV battery race, even though it's currently leading in the cost/capacity/density race. Lifecycles even as an EV battery are growing year on year, but it's too early to say an solid state sodium battery or some other battery technology won't someday replace it in a very short amount of time once the technology is proven.

      2. Doug Piper | | #6

        Embodied energy is not NECESSARILY polluting energy. Even more recent prestigious papers look at the declining carbon footprint of lithium battery production...which is what matters to me. https://www.ivl.se/download/18.14d7b12e16e3c5c36271070/1574923989017/C444.pdf

        Your argument is analogous to those who point to carbon intensive electrical grids in some places as an excuse for carbon intensive transport in all places. Not persuasive. Sorry.

  3. User avater
    Paul Kuenn | | #7

    If you are within 20 miles of work or play, you should be on a bicycle! Yes, slower with fat tires in winter snow but more of a great workout. Don't need any other way to stay healthy. PK

    1. Mike Cellini | | #8

      This is a disingenuous solution to a very real problem. I consider myself an above average cyclist and more comfortable than most on the roads. Even in my modestly sized hometown, where nothing is more than 12 miles away from my home on the west outskirts of town, I wouldn't feel comfortable bicycling to more than a handful of establishments.

      The truth is that most of our cities and towns were built assuming cars and buses (and trains if you're in a big city) as the primary means of transportation. Riding my bicycle across town would be stressful, hazardous, and time-consuming. I can't even imagine toting along my infant daughter. All this, in a town that is actually actively trying to improve it's bicycle friendliness and has an extensive trail system and bicycle "lanes".

      Busy, multi-lane state highways are intimidating to cross and downright terrifying to ride on even for short distances. These are often, at best, difficult to avoid (adding miles to an already slower form of transportation) or, at their worst, necessary to get where you're going.

      Electrification of our transportation segment combined with cleaning up our electrical grid is the only realistic solution we have in front of us. Bicycling to work is nothing more then a supplemental solution in our current reality, even if it is the ideal solution in an ideal reality.

      1. Expert Member
        Malcolm Taylor | | #9

        Mike,

        You make a lot of valid points, but I'm not sure disingenuous is a word I'd pick to describe Paul's post.

  4. Eric Habegger | | #10

    I think some of the arguments to this article are detracting from the really fine quantitative points made by Bruce Sullivan. There seems to be a non-appropriate hangover in the comments from a good article Martin wrote about saving energy causing you to use more of it. That's true. It can't be disputed.

    But some individuals are being sidetracked by that to think that any improvement in efficiency will ultimately be bad. This is a really, really bad take on Martin's article and is just an excuse for the temperamentally inclined nay-sayers. As others have said in not so few words "it's the carbon, stupid!" There's many ways of powering industrial and transportation processes requiring energy that do not contribute to GWP. The absolute best is to use the energy from the Sun. Wind, solar, and even water behind dams are all powered ultimately by the sun.

    Bruce offered a brilliantly succinct, understandable, and self effacing remedy for many to eliminate the global warming potential of driving. Why can't people applaud it instead of offering the usual human reaction of dismissing it and minimizing it in the form of saying, "well, you shouldn't be driving?" It strikes me that the author may be doing the equivalent of throwing pearls before swine. At least maybe some of us.

    I'm sure this sounds very harsh. Maybe it is overly harsh. I just hate to see important contributions by individuals being drowned on the internet in a sea of noise. It happens too often when people don't exercise proper restraint. I'm sure I'll be accused of not using proper restraint myself. But I wouldn't be happy if this wasn't spoken. Bruce Sullivan said something very important here and I have no connection with him. It isn't being said to prop myself up.

  5. thrifttrust | | #11

    There are reasons the Tesla battery factory is in Nevada. One of them is sunshine. On site PVs will power the factory by day to produce batteries that will store power to run the factory at night. (The machine that makes the machine that makes the machine.) What does this do to the carbon footprint of batteries?

    Traffic congestion wastes huge amounts of fuel. The city mileage of ICE cars is less than when they are driven at a constant speed, even high speed. I suspect that it's much worse in LA. EVs get better range in city driving because they are going slower. Most of the energy of acceleration is recaptured when slowing down, and no energy is used when idling. ICE cars are introducing start/stop technology to turn the engine off when stuck in traffic or at stop lights and signs. However, many find the delay and shudder of restarting undesirable. The solution? A battery of course. Hybrids don't start the engine till after the battery has gotten the car moving and like a BEV they recapture momentum.

    Tesla and others are working assiduously to reduce the environmental and social cost of battery manufacture. The Model 3 battery uses an order of magnitude less cobalt than some designs. It appears that before long solid state batteries will reduce the environmental impact further. Eric sites Tesla's Faraday acquisition, (I think he means Maxwell,) who's dry film technology should make batteries safer and cheaper to produce. By supplanting exiting processes it will reduce the strangle hold Asian companies have on battery manufacturing intellectual property. Tesla is bringing vertical integration not seen since the Ford Rouge plant. (Iron ore in, cars out.) to auto manufacturing, with attendant lower prices and higher margins. They want to do the same with batteries. It's all about scale. We need EVs to spur demand that will bring down battery cost.

    As mentioned, car batteries can have a second life in site and/or grid storage. They may have even more impact while they are still in the car. Cars average 30 miles per day, but consumers demand an EV have 300 miles of range. This represents a huge pool of potential energy available for grid storage whenever they are plugged in.

    1. Eric Habegger | | #12

      Yes, I meant Maxwell :)

  6. Debra Coleman | | #13

    Those who enjoyed this article might like the new book "Driving to Net 0". You can get it on Amazon. I met the author at the American Solar Energy Society conference and ended up beings one of the volunteer reviewers, but I have no financial interest in it. I found it very inspiring and educational on the many options that others have taken to power their vehicles.
    https://www.amazon.com/Driving-Net-Stories-Carbon-Future/dp/0692143831

  7. Jeff Towson | | #14

    Either way, it was very nice of them to repurpose an old Golden Corral into a house.

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