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Solar-Power-Augmented Prius Takes the Grid Out of “Plug-in”

Lapp’s PV Prius

A Canadian engineer has prototyped a Photovoltaic Prius—a 2001 Prius augmented with roof-top solar panels and an additional battery system to supplement the charge in the original equipment NiMH batteries.

Steve Lapp’s PV Prius is still a rough prototype—a demonstration of concept—but even with the limitations of the systems, he has achieved an initial 10% fuel efficiency improvement from 4.5 l/100km (52 mpg US) to 4.0 l/100km (59 mpg US).

From the original description of the plan:

...the fact that [current Toyota hybrids] can run on electricity alone, with their gasoline engines off, offers the opportunity to provide them with more electricity and therefore drive further with the gasoline engine off.

Electricity can be provided from the electrical grid by charging an onboard battery, and depending on where that electricity comes from, it will have various emissions associated with it. [The plug-in concept.] However if it is provided from renewable energy sources, such as photovoltaic panels, then it is “green”.

This begs the question of why not put the PV panels directly on a hybrid car and generate electricity onboard while the car is parked outside, or even while driving. The general reaction of people to this idea is that there could not be enough energy striking the roof of a car to provide enough electricity to drive any meaningful distance.

This is where the incredible efficiency of the hybrid car must be taken into account. To drive a hybrid car about 1 km, takes about the same electricity as to light a 150 watt bulb for one hour! The point is not to drive the car using only solar power, but to effectively use solar power to improve gasoline fuel efficiency.

How much gasoline can this photovoltaic hybrid car save? Well let’s look at the energy available from the sun on the roof of the car. For June and July in Kingston Ontario, about 6 kWh of energy from the sun strikes each square meter of horizontal surface. If we install 2 square meters of photovoltaic panels on the car and we collect 10% of the energy from the sun as electricity (well within present PV efficiency), we can theoretically go about 8 km each day on just the sun’s energy. If we drive 24 km on a sunny day, that is enough to reduce our gasoline consumption by 33%. This would take the Prius from 5.0 l/100km to 3.3 l/100km.

The PV Prius uses a 12-volt PV source with a small lead acid battery and battery voltage controller, inverted to 120 VAC, transformed to 345 VAC, then rectified with current control to nominal 300 VDC. The charge from the PV batteries flows into the Prius hybrid battery when the ignition is on.

The 300 VDC output of the solar subsystem is attached to the switched side of the original Prius battery, so the PV battery cannot recharge the NiMH while the ignition is off. The PV system can inject a maximum of up to 2 amps continuously into the battery while the ignition is on.

Lapp’s modelling predicts a 10%–20% fuel efficiency improvement for the 270 watts of PV (to be bumped up to 360 watts with the additional of a fourth panel), so the 10% on the first trip with little optimization was “a pleasant surprise”.

He is working with between two to six 20 Ah sealed lead acid batteries, experimenting to discover useful amount of buffer storage, given typical solar and driving conditions.

The decision not to charge the hybrid when the car is off was a pragmatic choice, given the financial and time constraints of his project. Among other issues, there would need to be a thorough analysis to determined the optimal PV-NiMH energy flow/charge relationship.




The performance of the Canadian prototype speaks for itself. Maximum Pb battery capacity of 120ah @ 12VDC would weigh ~80 lbs. The same capacity in Li-Mn would weigh ~40 lbs. 700Wp of conventional, glass face, aluminum frame PV would fit on the car and weigh ~ 160 lbs. Aerodynamics and weight would be inconsequential at typical US/Canada speeds. Plug in Hybrids with grid interactive inverters might be more practical with widespread PV. The most significant point is that PV right now costs no more than US$2.50/gallon in the energy equation and is much more environmentally benign.

Lance Funston

In the 1980's I saw a Saab prototype car with solar panels integrated into the roof for the express purpose of running a ventilation system while the car is parked (in the blazing sun of course). Though I was a high schooler at the time, the small sidebar made a big impression on me.

That small innovation would add little weight and cost to a vehicle reduce the need for the AC at start-up. It would also help ensure the car didn't run down the battery as long as it was parked in the sun.


I test drove the Prius and thought it was really nice! Very quiet and seemed to have pretty good power.


The solar idea is a great idea...in the desert SW. Here in WNY where the lakes make there own weather all on their own clouds win most months. The cold won't help battery life and power output either. How much did that "roof rack" cost anyway? Will it work with my mtn bike and kayak up their? What's the rollover crash results look like with all theat weight up their? Hitting a moose in Canada would have that down the road 50-75 meters down the road in a ditch. I do applaud the try. Kuddos....we've had enough Tuckerism in the auto industry.

I'd like to offer another off-grid way to get electricity for the Prius. Why not use a magneto? How many of us still have a petro lawn mower that runs on a magneto? One or several magnetos could be placed on any rotating axle shaft. As long as that baby is rolling.....it can be back charging that battery pack. I'll leave it up to the engineers to figure out the particulars.....but it would work. No loss in drag, no hail damage, no kids pointing at street corners, neither heat of day or dark of night....postalman's motto. The big advantage I see it's always working while you are rolling. No expensive regenerative DC motors and controllers, no expensive PV equipment, really really cheap.

Most of these elec hybrids work really good below 30 mph and then they need a gas engine to keep them going overcoming drag and friction. If you were to use this magneto technology it just might make them all electric up to 40 or 45 mph before you need a gas assist and increase their range significantly. That'd make them
much more marketable.

Let me know how it works. I to am concidering trading to something that uses less fuel. I haven't seen anything do-all for a family with 2.4 kids as yet....I'm hoping the EPA/Ford hydraulic hybrid comes to market. The Tempo powered model Expedition got 28mpg and the VW TDI model got 33 mpg. Now put a Prius elec motor under the hood and magnetos on the axles and you've got something......I'd replace the current Explorer...with exactly that.




The real value in a hybrid/solar combo is not in car rooftop PV cells. Why use PV on a car where the angle and other factors are not optimal? Solar on garage rooftops, or better yet concentrating solar PV farms (Boeing claims a new PV cell that functions at 300+Suns) fed into the grid is far more efficient. The real value of huge fleet of rechargable hybrid cars would be in the huge amount of electical stoage capaicty they represent. Stoage capacity is the bane of almost all renewable energy. Intertie the cars to the grid with an internet connected power controller on the car to suck power off the grid when the sun is shining and the wind is blowing. Sell the power back when it is cloudy or grid demands exceed supply. This Solves both of the big problems with each system by maximizing the value of both.

Jan de Boer

A lot of the comments mention the wheigth and aerodynamics of the photovoltaic panels. It should be noticed that this particular car is just a proof of concept, made from off the rack parts.

Most of the weight of the panels is the glass pane. The solar cells themselves are very thin and thus very light. In a future version the cells could be stuck to the roof and hood of the car directly, and covered by e.g. a teflon coating. In that case the extra wheight would be negliable and the aerodymics would not be influenced.

Another issue mentioned is the energy-payback-time of photovoltaic panels. It is true that it is currently releatively long (1.5 to 4 years). But it will more than pay itself back during the lifetime of the car. Moreover, the photovoltaci technique is developing rapidly. The current shortage of silicon makes manufacturers very inventive, so they use less and less material and thus less energy in producing the solar cells.

Sabin Speiser

12000.00 miles a year average driving
1000.00 miles a month average driving
36.63 theoretical kwh in gallon of gas
0.25 efficiency factor of internal combustion engine
9.16 actual kwh in gallon of gas
0.13 kwatt/m2 of solar cells at peak insolation at 1kw/m2 and conv. Eff. Of .13
8.54 m2 dimensions of a Camry's top surface
1.11 kw peak power from cells covering a Camry
5.00 hrs average peak insolation US ( # of hours where the sun deliver 1kw/m2)
5.55 kwh per day from solar cells on car surface
0.88 efficiency of solar energy from cells to wheels
4.88 kwh/day of solar available (total kwh times efficiency factor for electric cars)
0.53 gallons of gas (kwh per day from car surface / actual kwh in gallon of gas)
32.88 miles a day average driving (miles per year / days per year)
23.00 mpg for car
1.43 gallons/day for an average car
0.40 kwh/mi for an average car (actual kwh per gallon / MPG average car)
0.19 kwh/mi for GM ev1
29.20 per day from Sun for GM ev1
3.00 dollars/gallon of gas
1.60 dollars a day
1565.22 $ in gas per year for average car
800.00 $ in gas per year for hyrbid
89.46 $ in gas for solar hybrid (miles per day driven – miles available daily from solar cells time days per year / MPG of hybrid)

One could object to any one of the numbers, but it takes a whole lot of tinkering to make this plan a loser. If and when gas creeps up to $5 a gallon, its going to look even better.
If 40% of our oil use is for cars, using technology like this could conceivably cut our oil use by 30%. Think about it.

A Brietzke

The idea of putting solar panels on garage roofs is great, but is neither here nor there in terms of making a fuel efficient car. Yes, we should have solar panels on both our garage roofs and our house roofs but not only to charge plug in solar hybrids but to have a hybrid power system in general. As the panels become cheaper and more efficient this will become a no brainer, and neighborhood roof tops will be solar panelled at an exponential rate. I know there's a lot of naysayers out there who like to put down green energy in general, but you will be proven wrong in the long run. Oil is less then two decades away from virtual irrelevance. The true reason wars are being fought over it is that big oil wants to suck as much of it out of the ground a.s.a.p while it's still worth something. I'm sure many will refute this, but in less then a decade when every single car made is a hybrid of some sort and every house has both energy efficient appliances and a hybrid power system, you'll see how obvious it was all along.


I am printing out a lot of this good stuff and posting it in laundry rooms and tac boards all over town to get the message across. It's good fun and good advertising too. I like to do no nuke advertising.

Richard P. Gunion

I think that if the major auto manufacturers researched this more they could come up with more cost effective and better solar panels. Maybe the idea of a PV paint is a good idea-red paint maybe? The solar panel does not help the styling but lets say the solar panels could be flat and flush with the roofline and put on a car like the Honda Civic. The panels themselves could be styled in some cool blue finish. Some people like the styling of the Civic better than the Prius. I think it is worth some money to research even though some say solar panels do not pay for themselves. Maybe improved materials can be found and economy of scale could bring prices down. When gas hits $6.00 a gallon maybe those solar panels will be cost effective.

Paul Risberg

Many of the comparisons above fail to address a major issue. Why do we want to use less petroleum/fossil fuels? Political ramifications of industrial dependence? Adverse environmental consequences of extraction and combustion? Relative expense of conversion to usable energy? Most of us could agree to pick at least one of those arguments. I would ask you to consider the merits of reducing consumption through efficient application of capital. That is how a movement obtains power; it's what will work best.

Putting solar panels on a car is not an efficient use of capital. Charging your cars batteries, (even though they will not be very efficient) with grid or renewable electricity at your home or workplace is a far superior way to reduce your fuel consumption per dollar invested. So adding storage capacity? Very Good. Turning your car into a moving solar array? Not too smart. Angle of Solar incidence (insolation) will never allow even a small percentage of production cited above.
But find a way to tie your home or business solar production to the grid, so you and your neighbor and me can create a usable energy network. Capture thermal energy and use it to heat your water or your home, maybe even cool it soon. Carpool to work. Buy efficient appliances. Drive 55 mph. My point is to not lose sight of the real goal, which is to reduce consumption of fossil fuels which are proven to have negative consequences. Debate the specifics of those consequences if you like, but if you use less, you impact the demand side of energy supply. That, my friends, is the only side that holds the cards.

Kirsi Enckell

I would love to do this to my '03. However, I would probably consider using flexible PV. I have also wondered why golf carts used on have yet to do this. Since there are so few trees on a golf course they would get quite a lot of sun daily and could virtually eliminate the need to plug them in every night. I would think that would be very attractive to the clubs, and would prbably pay for itself in short order

John Adams

If I could get 5 miles per day on just one day's charge of my solar panels I could have a second car and drive each one on every other day. This would get me to work and back each day never needing to buy gas. I already drive two cars, this might be the way to go.

D. Ünsal

imagine year 2010 maybe 2015. brent crude exceeding 150 usd (8 usd gallon), inexpensive solar panels with efficieny exceeding %50. lapp's work WILL make sense.

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Peter Hubbe

Thanks, Steve! I have a 2005 Prius. Yours is a step in the how do we get from here to there.
Shirley E., I am a believer in the postal truck solution. Keep advocating.
I spoke to 70 5th graders today and helped them get started with Jr. Solar Sprint cars. Let's pray that they take our energy challenge on.

Jay Fidler

This is alot like a Drag Race. At this point I think everyone, self included has checked the tire pressure, folded down the antenna, windows up and A/C off (most of the time) even speed up and slow down using the cruise control... So anything that will give me another 10% or 4% I want. If I can't buy it then I'll design and make it myself. Why? Because 30 years from now I expect to be driving the same '03 Prius and I'll look back and say 'that was the smartest thing I ever did (carwise)... My '03 has about 27k miles is almost perfect, has overall average of 54.7mpg as of today and wish I had 2 more (for kids)... Best run = home to grocery to home 17.5 miles 77mpg. Worst run = San Antonio to Rio Grande bucking a strong head wind 44mpg (note, car topped out at about 104 so it hard to keep up with traffic). Another 10%...hook me up

John Chalotay

Tnis is ALL great, but did anyone mention the weight of the vehicle ? The 2000 kg vehicle carrying an 80 kg person! It is obvious that most of the energy is being used to move the 2000 kg vehicle from point "A" to point "B" How about reducing the 2000 kg to 1000 kg ?
How much energy would be saved ? Almost 100 years have been spent on improving the amount of energy stored in the gasoline. Does it make any sense to load your vehicle with heavy batteries ? One liter of Hydrogen stores about 3 times as much energy as one liter of gasoline. Which one would you pick ?


8.54 m^2 on a camry (why not a prius?)
A prius would be 7.56m^2 (175 in long by 67 wide)
But you need to subtract the area of the glass so you can see out, so the area availabe for the array will be less but we'll run with your numbers.

4"x4" typical solar cell
828 cells needed to cover area
$9.73 per cell
$8056.44 for array

4.88 Kwh per day

Prius battery pack
6.5 Ah 7.2V 28 cells

1.3104 Kwh capacity (too small)

3.5696 Kwh more capacity needed to hold a 1 day charge
77 more cells needed
1 cell made of 6 1.2V cells @ $15.24 each in 100 unit quantity
$7040.88 for battery pack

Total cost so far.

To save $710.54 in gas

4.88 Kwh /day
1781.2 Kwh / yr
price of electricty if purchased from power company
@ $0.10 per Kwh

intrest rate on 1 yr CD 4.43%
money earned if $15,097.32 put into 1 yr CD


Don Chambers

Batteries have been a big part problem. They are heavey, take long to charge and have a short life cycle. Toshiba recently announced a new Lithium Ion battery that charges to 85% capacity in ONE minute, is much lighter, smaller and lasts much longer. You could fill your batteries with electricity FASTER than you can fill Gasoline. Thanks to nanotechnology this is way more efficient.You could drive from Florida to new York just on electric. Just need a 60 second plugin along the way. I could imagine having 2 sets of batteries. One stays at home and recharges off your roof top PV panels all day. You come home at night and then transfer the electricity to the car batteries. Neuclear plants supplying non-fossil fuel to run the grid would help global warming also.

You will see other big companies like Sanyo and Panasonic come out with these batteries also. You will see fully electric(non-hybrid) cars and trucks by the end of 2006 in China and Japan and they will weigh much less than the heavey pigs you see today.

As oil consumtion begins to drop so will the price This will start end of this year.


Why do this to a Prius? Doesn't the alternator in an ordinary car have generate hundreds of watts anyway? You have to power the radio, running lights, climate control, water pump, fuel pump, computer, ignition, etc. That's gotta be at least 270 watts of load before even getting to the battery.

So, instead of trying to use PV to charge the batteries in a Prius, seems it would be more practical to use PV to run the electrical system in a Camry. Or am i mistaken?


From the main article: "For June and July in Kingston Ontario, about 6 kWh of energy from the sun strikes each square meter of horizontal surface."

It has already been pointed out early on in the comments, but I will say it again as it has not been updated: The sun gives out 1 kwh per square meter not 6.

It is unfair to give people the impression that solar power is 6 times as good as it really is currently (even though I love solar power).

brian k

Thank you, everyone for letting me know that I am not the only person thinking up "crazy" ideas like putting PV on a car! I live in Detroit (spelled INTERNAL COMBUSTION ENGINE) and this all seems very far off. But I am learning and hope to have an "off-the-grid" vehicle within 5 years. I even wonder about hood scoops with wind turbines that assist PV in recharging duties. Is this crazy, too?
Thanks, again.


One of the weak points of the Prius is that if you dont use it at least once a week the 12 volt battery (which if i understand correctly loses its charge because it trickle charges the "electric" battery) will discharge and you have to take the prius to a garage to reboot the computer. So you have to run your car for 10 minutes every few days. To me this is a big weakness of the Prius and very incovenient.

Surely the solar panels can offer a solution to that? wouldnt it be possible for them to keep the 12 volt battery charged up while i am away not using the car? anyone have any instructions on how to do this?


The 6 KWH/m2 = 1 July DAY of sun energy.
The 1 KWH/m2 = 1 HOUR of sun energy.
The article is correct.

This is a VERY informative thread. Thanks ! Hopefully humans will soon become as competetive about conserving energy as they are about almost everything else.

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