The Solar-Powered Prius

Solar-Powered Prius (Source:

At the end of my recent essay on Nissan’s electric car announcement, I wrote “For my next calculation, I need to see how much power I could generate by putting a solar panel on the roof of my electric car and letting it recharge all day.”

In response, a reader wrote and told me that a feasibility study has been done for this on a Toyota Prius. The paper was the source of the above picture of the prototype:

Prius White Paper

From the paper:


The major automobile manufacturers are producing hybrid automobiles, which are part electric and part gasoline powered. Could these automobiles take another step and obtain some of their fuel from the sun?

Solar Electrical Vehicles has developed a prototype PV Prius to help answer that question. The PV Prius is fitted with a custom molded fiberglass photovoltaic module as shown in Figure 1. Solar Electrical Vehicles has applied for a patent on the PV Prius solar system.

The photovoltaic module is rated at 215 watts at AM 1.5. The module is connected to a DC-DC converter and peak power tracker. The output of the converter is directly connected to the primary motive NiMh battery.

The daily power production available for charging the Prius primary motive battery is estimated to be between 850 and 1,300 watt-hours. The car uses 150-175 watt-hours per mile. Thus, the expected range per day that the PV Prius would have on solar power alone would be between 5 and 8 miles. Based upon a nominal daily trip length of 28 miles the gasoline consumption of the PV Prius would be reduced by 17% to 29%.

The following section was of particular interest to me:

Can a PV Prius obtain all of its fuel from Solar?

The answer to this question is a definite yes providing that the stock Prius, in addition to having the solar modifications described in the previous section, increase the size of the secondary battery and the DC-DC converter used to deliver solar energy to the NiMH battery. Using a maximum depth of discharge of 50% to provide some reserve power and extend the cycle life of the enhanced Lead Acid battery, the capacity would have to be increased from its present 3 kWh rating to 8 kWh.

In addition, the 48 to 240 V DC-DC converter capacity would need to be increased to at least 2000 watts. With this battery capacity, increased energy from a residential photovoltaic array could be used to recharge the battery at night when the car is parked in the garage. This complete system is the Total PV Prius.

How can this be? How can you recharge your Total PV Prius at night parked in the garage? The answer to that is net metering with your local electric utility. That is if you live in a region of the country which net metering is offered for residences with a grid connected photovoltaic array, then the owner of the Total PV Prius would be able to supply energy to the utility grid during the day light hours and have it returned to him in the evening. While the energy returned to the homeowner may be produced using fossil fuels, the energy supplied to the utility during the daylight hours would have reduced the use of fossil fuels by an equivalent amount.

Some of you solar guys take a crack at that, and let’s discuss what some of the hurdles might be. The economic analysis isn’t all that promising, as the expected gasoline savings over the lifetime of the vehicle is estimated at somewhere between 300 and 600 gallons. That’s not going to warrant much capital on a purely economic evaluation. No cost estimates are given for the required modifications, but I am guessing the cost is more than can be justified by the gasoline savings.

It’s a start, anyway. Keep in mind that this is a Prius, and a lighter car can have a much greater range. The economics may look a lot better if the range is higher, and so is your daily commute.

34 thoughts on “The Solar-Powered Prius”

  1. That’s impressive. The technology is certainly there to produce a vehicle that drastically reduces gasoline usage. This is the kind of thing I like, off the shelf technology that’s available today. Not some pie in the sky hydrogen economy nonsense. Oh, and that 175 watt hours/mile is much better than I was expecting. Unfortunately it’s just too expensive for most people. Maybe cars will become things that only rich people have, while the rest of us ride public transportation.

    One thing this country desperately needs is a national net-metering standard. This would make it much easier for people to install solar panels on their roof without the need for batteries. I don’t think my state has any provisions for that at all. If I wanted to install solar panels, it would probably have to be a standalone system.

  2. I’m building my own solar powered bicycle and I think the thing most people don’t realize is the power to weight ratio sucks. I’m using two sunpower-200 panels and they weigh 35 pounds (15 kg) each for 400 watts. Maybe someday thin film panels will be available retail but they aren’t now.

    My goal is to have infinite range as long as the sun is shining. The solar prius simply collects 5-8 miles worth of sunlight a day. Given the impossibility of cutting the cord, I think there are better places to put solar panels than on a car. Where it adds weight and has streamline problem.

    There’s a solar car race in Australia but those guys have a million bucks to build their car with.

  3. I am willing to bet that parking in the shade and not having AC saves more energy than parking in the sun and have the cabin go to 140F while charging batteries with PV. It probably takes more energy to cool the cabin off with AC after a day in the sun than the PV can generate in that day.

    If we are going to go down the rather silly idea of using PV to charge cars, at least put the PV on the roof of a carport and shade the car.

  4. I think Bob’s carport, or parking garage covered with solar panels is a great idea, and the cars plug in. However, a panel on top of the car itself is an interesting idea for anybody living in the Southwest.
    The whole idea is to get the car to handle most commutes and trips w/o gasoline.
    I entirely disagree that we have to give up cars (though I would like to see much more mass transit).
    The Volt, for 30k, is here in about two years. Maybe second-gen Volts will have rooftop solar panel options. You get an extra mile or two gas-free on your morning commute.
    That might be the difference between turning on the gas motor (which recharges the battery, not connected to drive train in a Volt) or not.
    My own take is that oil will not become scarce enough in the next 20years to economically validate a complete switch to Volt-type cars. The introduction of Volts and low mpg cars will depress demand enough that oil prices go down.
    Another thought: If the Volt works, then the huge wave of auto buying anticipated from China and India will not necessarily translate into more gasoline demand. More electricity demand, yes, but not more demand for oil. Those countries can nuke up, ala France, or go solar and wind, ala Germany. It is possible, France and Germany are doing it, not talking about it.
    The doomster scenario is receding already. Indeed, we might see cleaner and quieter cities as a result of EVs. Not so bad.

  5. This appears to be a gimmick to me. The solar panels are for show only. The panels can’t generate enough power to drive the car, even once it reaches cruising speed. What makes this work is hooking the Prius up to the grid during the day and use net metering to put power in at a high selling price and then purchasing it back at night.

    Why not just permanently mount the cells where they can be optimally placed to maximize power generation. Then you wouldn’t be hauling the weight of the cells around.

    I’m a proponent of real time pricing and net metering. It looks like here in Texas we will be paying around $15 /MWhr retail for 12 month power contracts. If wee had net metering night time power might be as cheap as $5 with daytime at $25 or more. That would make solar have a lot faster payout.

  6. Back in 1998, when I was visiting my sister in Las Vegas, I couldn’t help thinking that an solar-enhanced electric car or shuttle van would work there. Probably better than almost anywhere else.

    High population (1 million+) but low height buildings due to zoning (limited to 2-3 stories, if I was told correctly, excluding casinos).

    Considering everyone drove there, most driving was less than 10 miles, lots of sun, and low buildings I imagine Las Vegas would be ideal for a pilot program.

    But after reading this blog and it’s comments, I think a carport might work better.

  7. Nice as an incremental change. As always I want to point out that any scheme that relies on using the grid as an energy “bank” is inherently limited in terms of what fraction of overall energy consumption can be shifted to renewables. I know you (RR) understand the energy storage issue but I fear the general public and (apparently) many political leaders do not. I continue to think that taking steps like this, while good, aren’t going to be enough to head off a CO2 disaster if we keep using coal for baseload electricity. In parallel we should be replacing coal-fired capacity with nuclear, at least until we’re certain we’ve got a commercially viable power storage solution.

  8. Solar Cars?
    I keep worrying that we are about to see a huge price dump in oil, which will crush alternatives and conservation, just like in the 1980s-1990s.
    According to MasterCard, USA gasoline demand is down more than 6percent y-o-y this year. see
    No more shortages etc. Remember RR’s handwringing last year at Memorial Day that we would run out of gas?
    And next year, as new, low mpg cars enter the fleet, will show more declines. And the year after that, and so on.
    When, in 2010, USA gasoline demand is down 18 percent from 2007, will prices stay firm?

  9. 1st – PV is expensive now, due to a scarcity premium (relative to skyrocketing demand) which is necessary to ration supply. PV won’t be the most cost-effective way to get electricity for some years.

    2nd – an expanded battery (combined with a plug) for a Prius definitely would pay you back, assuming normally priced electricity.

    At 45MPG and $3/gallon, a Prius costs 6.7 cents/mile.

    Now, good quality cobalt-based small-format batteries, as used in the Tesla, cost $400/KWH. Iron-phosphate is less expensive, and large formats are less expensive. The plug-in Prius is planned by Toyota for 2 years from now, which gives us another 2 years of the normal 8-10% annual cost reduction seen with li-ion’s. Large scale PHEV battery production will instantly raise the volume of production for these 2nd generation li-ions to very large levels compared to conventional li-ion, reducing costs further. That gives us a reasonable forecast of $300/KWH (if this seems too aggressive, perhaps you’ll grant that this is very likely several years later, when PHEV’s have gone beyond early adopters, are ramping up to much higher production volumes, and batteries are that much more mature).

    A123system’s batteries can handle 5,000 discharges at 100% depth of discharge. If we assume 250 per year we have a battery that will last the life of the car. At a 10:1 capitalization rate, we’re paying $30/KWH per year, for 250 discharges, or $.12 per KWH discharged.

    At .25kwh/mile, that’s $.03 per mile, less than half the Prius cost. If we double the battery size to account for GM’s conservative decision to only use 50% of the battery capacity (this is similr to the Prius, and almost certainly unnecessary, but GM’s taking no chances at all), we’re still at $.06/mile. Toss in $.02/KWH for night time electricity, and we’re at $.065/mile, or rough parity.

    Of course, taxi’s and other fleet operators are likely to recharge more than 250 times per year, dramatically raising payback. For the average driver, add in CO2 costs, and other external intangibles like independence from the ME, and the ability to weather gas shortages (as you note), and you have a compelling case.

    And that assumes $3 gas.

  10. Outfitting the roof of a hybrid with solar is an excellent marketing strategy. It’s much less about the actual solar contribution, more about making the vehicle as eco-friendly in appearance as is possible.

    There is one small energy advantage to panels on the roof (to whatever degree they can contribute). Solar energy collected while the vehicle is in operation will go straight to the motor with only charge regulation and I2R losses, bypassing the much more significant battery charge/discharge losses.

    Of course if the vehicle is being driven to and from work in the morning and evening, the sun angles will mean little on-demand solar contribution, perhaps 30% of peak midday. The rest of the solar day would go to charging the vehicle’s batteries.

    I don’t know what real life charging efficiency of the EV battery designs really is. As with lead-acid, I imagine it varies greatly with charge/discharge rates, current state-of-charge, and temperature.

    I remain intensely skeptical that >90% overall watt-hour charging efficiencies are being reached in actual use. Keep in mind that when battery people refer to charging efficiencies, often they are referring to the ampere-hour efficiencies, not watt-hour. There’s a big difference. We’ve always been told to budget a 75-80% overall watt-hour efficiency for lead acid, even though the ampere-hour effiencies are about 95% usually.

  11. For round-trip charge-discharge efficiency: NIMH is about 70%, and Li-ion is about 90%.

  12. Now that I think about it, the “Solar-Powered Prius” is not a marketing gimmick, it is dishonest. The majority of people who see this will believe that the solar panel runs the car. As in, you don’t need gasoline, the solar panel puts out enough power to drive the car down the freeway.

    If you were lucky the solar panel might have enough juice to operate the radio.

  13. Have you guys seen this car:

    “Surface area : 3.6 sqm of photovoltaic cells (yield 21%)
    Power : 600 W
    Average solar contribution: 18 km/day”

    Even with this vehicle being basically a solar panel on wheels the majority of the power comes from plugging it in. I wonder if you put thin film solar over, say, the roof and trunk of a Tesla, how much would it contribute to running the A/C?

  14. “Now that I think about it, the “Solar-Powered Prius” is not a marketing gimmick, it is dishonest. The majority of people who see this will believe that the solar panel runs the car.”

    King, the same has been true for whole-house solar for years. People think they can heat their house with PV, when in fact the PV is only running the high-quality power demands: appliances, computers, lights, etc. I weary of delivering the bad news sometimes.

    Yes people will be duped. On the other hand, early adopters of alternative technologies, those with disposable income, often don’t care about these things. Many of us selling solar preach conservation before offseting with alternative clean power generation, but conservation isn’t sexy.

    Whatever our motivations, we Americans choose to display our values through what we buy. That being the case, there will always be ready enablers, no matter what the values are in question.

  15. Can anyone answer this question for me?

    I’ve often heard the statement that PV cells never generate more energy than was used to make them. Is it a myth or fact? I’ve never seen any reputable source either way.

    Mr Anonymous

  16. anonymous: It’s a myth. The panels last for decades; the energy used is paid back within a couple of years.

    Try this for a start. That was eight years ago; efficiencies have probably improved a great deal since then.

    I’m no fan of solar panels; the irrational love affair some environmentalists have with them has proved an unfortunate distraction from much cheaper renewable options. But this particular myth is wrong and damaging.

  17. “According to MasterCard, USA gasoline demand is down more than 6percent y-o-y this year.”

    And Chinese demand is up 7%,as usual. That’s an extra 500,000 bpd each and every year Benny. China’s gas is frozen at $2.49 per gallon. Don’t expect much demand destruction from Venezuela and other oil exporting countries either. Would you cut back with 15 cents a gallon gas?

    I think half the world’s population will starve in the next 25 years. Another “Maunder Minimum” is underway,and this planet can’t feed 6 billion people under those conditions. Even that won’t create much demand destruction though. The half that survives will need twice the heating oil to make it through winter.

    I’d take the 8 miles the solar panel provides. I don’t travel 8 miles most days. And 600 gallons is 2400 bucks at current prices. Over the life of the Prius,it could add up to $10,000 or more. Oil and gas will only get more expensive. Food will get ridiculously more expensive….and soon.

  18. It looks like here in Texas we will be paying around $15 /MWhr retail

    I think you’re off by 10x. Anyway, not all of TX pays the same price. San Antonio CPS charges a dime per kWh.

    A123system’s batteries can handle 5,000 discharges at 100% depth of discharge.

    At the high power required for PHEV duty cycle? Do you have a link to that spec?

    Also A123 is charging a LOT more than $300/kWh. What’s your source for that price?

  19. Maury-
    But the US consumes one-quarter of the world’s oil, and China about 7 percent. Addtionally, the developed world is using less, not more, oil every year.
    The only real grwoth in oil use is in oil-exporting nations, and China. And China has real energy programs in place to move it towards self-sufficiency within 5-10 years. Check out world oil stats on the British Petroleum website.
    I do worry that another Ice Age is on the cusp. You might want to buy land just a bit south of the equator. I would suggest Thailand, or the far north of Australia.
    By the way, did you know that when man first arrived in North America, the sea levels were about 60 meters less than today (14,000 bc)? Beachfront land might make an interesting play — just make sure to extend your dock out every year, and add additions on to your house towards the beach. Keep it up for several centuries, and your ancestors will be land barons!
    by the way, our ancestors must have been tough as nails. Can you imagine crossing the Bering Strait in an Ice Age? On foot?

  20. anonymous: It’s a myth. The panels last for decades; the energy used is paid back within a couple of years.

    Oh? That’s why they are powering polysilicon production with PV, building fully automated PV plants and we are seeing a constantly decreasing PV cost. Just like we are farming with ethanol powered tractors producing so much renewable fuel that we don’t know what to do with it.

  21. Benny, China is up to 9% of world oil consumption. They gain almost 0.5%/year. They are not moving toward “self-sufficiency”. They have a goal to reduce consumption per unit GDP 5%/year. But GDP grows 10%/year so even if they meet their goal their demand still grows 5%/year. And they are falling short of their goal, so demand growth is more like 7%.

    Demand growth from oil exporters will not stop as long as oil prices stay high. They’re drowning in cash. The developed world has to do more than cut back, we have to cut back enough to overcome this positive feedback. So far we are not. Maybe 130 will do the trick. Maybe not.

  22. I think you’re off by 10x. You are correct, should have said $150/MWh.

    Not all areas of Texas are under retail price deregulation, city owned and coops could decide for themselves whether to opt-in. Nearly all didn’t. The city of Austin is one example.

    Natural gas still sets the clearing price for power, primarily during the day. That is why we need time of day and net metering. Nuclear and coal dominate at night and can produce at a much lower price.

  23. Benny,my dad planted his garden mid-April,like he’s done for the last 40 years,and it was killed by frost. He doesn’t remember that happening before. He lives 70 miles south of St. Louis. If another Maunder Minimum is underway,growing seasons will shrink to almost nothing in the worlds most productive farming regions. Solar Cycle 24 was supposed to be more active than average. And it was supposed to be underway 18 months ago. Not happening. Cycle 25 is projected to be extremely weak. The beginning of each century usually brings weak solar cycles and global cooling. But,some experts think we’re at the beginning of overlapping minimas. It could get really chilly,really fast.


    interesting improvement of photovoltaics.

    Also, I really can’t understand why people piss all over air-conditioning (justifiable energy use) yet don’t get cranked over wasted energy with outdoor lighting (20 billion $$ worth per year).

    in my teens, I worked for my father and spent entire summers covered head to toe in dirt and grease so thick that 30 minutes in a shower scrubbing with lava soap only get most of it off. You don’t know what hot is until you’ve worked outside in a dirty industry carrying hot pieces of metal and wood that had been sitting in the sun all day. I want air conditioning for the rest of my life thank you very much.

  25. Speaking of consumption, can anyone explain why VMT is dropping (-4.3% this March vs. prior year) and MasterCard is reporting gasoline sales 5-6% below normal, yet DOE Finished Motor Gasoline consumption numbers show a 0.5-1.0% drop? Are the DOE numbers somehow missing a huge drop in gasoline consumption? What gives?

  26. Tis good that alternative technologies are being employed in transportation. Also good that folks are debating what technologies are the most appropriate. The silly part to me is the debate over car ports and Air Conditioning for anti-social single occupant boxes. Why not instead work on correcting the root problem, poor infrastructure. Is anybody working on stopping future building of or correcting existing stupid suburban infrastructure problems. Relying more on trains than trucks for freight, increasing access to public transportation, adapting the normal civilized work standards and schedules to our progressive reality? Anybody have an air conditioned solar powered jet pack or George Jetson UPS truck, bus, garbage truck, etc, to efficiently navigate the stupid fenced in courts, culdesacs, ways, parkways, etc?

  27. Bob Rohtenhansky: what do you call this other than a fairly strongly declining cost – particularly when you compare it to the trajectory of virtually every other energy source out there right now?

    It’s still got a long way to go to be cost competitive with other renewables, let alone fossil fuels, though.

  28. Hello Robert,

    Appreciate your thought experiments and calm demeanor in discussing the issues.

    What follows is purely speculation based on trends I’ve observed so far…

    Solar cars are a definite part of the future for multiple reasons, not the least of which is continued downward trends in prices.

    Question first is, what is a solar car? In the beginning it will act as a buffer. But as prices go down and efficiency edges up the key question becomes how much more wattage can it create and give back to transport?

    What are the scales or models for future integration, power and price?

    Then there is future look; where solar glass will produce energy as chemical giants and nano-tech materials companies enter the market. You know things are cooking when Dupont and Applied Materials are in the race.

    Everything from paint to the windows will be an energy net positive producer.

    Parking garages are practical as mentioned and are being pursued rapidly.

    But individuals are well, individuals. They’ll want alternatives for a drive thru a park or running their digital lifestyles where large scale solar systems do not exist or no plugins are available. Thin film, Window embedded solar, paint nano tech, will expand the capability to make the entire car a giant mobile energy capturing device in the future.

    It will be practical too, as a solution so you never get stuck again for more than a few hours. Solar can be a saftety net in the future. Essentially never running out of “gas” again.

    That is a few practical reasons to put solar on and in the car.

    Integration is a matter of time and price. With high tech people from silicon valley and other chip engineering companies, breakthrus in solar will continue down like the price curve of computers every year with e-log scales of prediction long term.

    To addd for discussion…

    Solar Windows

    Astrolab by Venturi

    Future Do it Yourself Ink-Jet Solar

    Paint your own creative Solar Cells

    Now… if we think about future tense, it makes sense, espcially when more genetic breakthrus utilizing photosynthesis and the nano-power machines inside of us and nature like ATP Synthase. Nano-motors are a future too in parallel development with solar.

    The future is here in a small way already in labs and prototypes. Commercialization and price reduction are key issues, along with upping the energy efficiency of all these new products coming on board.

    In the future, every sq nanometer of our lives, from homes to cars, buildings, highways, trains, planes, backpacks will be energized by the sun. Solar highways will replace hot concrete spots. We will develop methods to cool cities thru infrastructure like grass and earth are natural absorbers. Looking now into the future we can see how inefficient we’ve been living on the nano level of life.

    I may seem overly optimistic. But based upon breakthrus and nano-tech research, I believe I’m being fairly realistic. Solar cars will be in the future.

    Maybe one of the engineers can give us a look at semi-conductor price history and do a comparison to eventual solar price modeling predictions along with energy efficiency enhancement. Certainly the price list is out there somewhere already?

    What would scales be if improvement rose from 12-20% up to 40% efficiency in the future? Then apply it to windows for more coveage and nano-paint for future reference. You essentially have the current solar car in Australia with a 20% efficiency improvement. How many years until 40% efficiency rating?

  29. Here are PV price trends in Japan from 1993 to 2002

    Japan PV price install trends

    2006 Consultant California study

    If manufacturers can automate production of nano-materials in windows and coatings on cars, prices can drop dramatically in the future. Especially if silicon usage drops.

    Big if?

    50 percent price production in 2 years?

    “Solar PV prices declined slightly in 2007, with greater reductions held back by the pace of demand and a continued shortage of polysilicon. Analysts expect more dramatic price drops, possibly as much as 50 percent in the next two years, as more polysilicon becomes available, production and installation are further scaled up, manufacturing efficiencies increase and more advanced technologies are introduced.”

    I’d be surprised by 50% reduction, but then I’m not an expert or insider. The trend is definitely downward. The key is does technology transfer to cars? Yes, eventually, yes.

  30. what do you call this other than a fairly strongly declining cost – particularly when you compare it to the trajectory of virtually every other energy source out there right now?

    Any idea of the reason for the rise from about 2003-04, before flattening? I am just wondering if that’s something cyclical, a one-off, or it’s just a result of demand.

    Thanks, RR

  31. Robert: Short answer is I don’t know; maybe others do.

    Longer answer: it may have something to do with the increase in demand from Germany and Japan for solar panels because of the introduction of feed-in laws. This led to a shortage of pure silicon, which used to come from the stuff that was rejected for making integrated circuits.

    I know that this happened, but I’m not sure whether the timing conicided with the price spike.

    Maybe the people responsible for constructing the price index will be able to tell you more.

  32. Any idea of the reason for the rise from about 2003-04, before flattening?

    It’s the polysilicon shortage. PV historically piggybacked off the semiconductor industry, using QC reject and factory overrun wafers. This worked great when PV silicon consumption was a fraction of the chip industry’s. This situation reversed by 2005 — due to rapid growth the PV industry now consumes more silicon than the chip guys. The tail is wagging the dog, so to speak.

    By about 2003 PV consumption was high enough to drive polysilicon pricing and by 2004-05 the situation got really serious. This spawned a lot of “solar-grade” polysilicon factories which are now starting to come online. In theory a solar-grade factory will use much cheaper processes and produce really cheap wafers. We’ll see. If the planned factories all come online PV production will soar. We might even have a glut.

  33. Maury-
    I don’t know if you are still reading, but there several websites which contend that the last year, 2007, was 0.75 degrees cooler than than 2006, a very sharp one year drop.

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