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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.




How heavy are photovoltaics? It just seems like it'd be inefficient to lug them around, both due to their mass and their less-aerodynamic shape.

In general, it seems like it'd be more efficient to keep the PV where the car is to be parked during the day, plugged in.

Still, any improvement is good, so this is good. And, it'd be particularly handy for vehicles that don't park during the day, such as delivery vehicles (which have huge flat roofs), taxis, and big rig trailers.


bravo, keep up the good work!


The thin film panels dont weigh much at all and they are areo enough being flat and thin. In the end if you realy went all out you would glue the think film right to your roof thus elimainating any wind resistance issues.

And it does protect you from running out of gas;/


Ideally, the PVs would be integrated into the roof...but that was beyond the resources/finances of the project. :-)

Shirley E

A few months ago I was reading somewhere about PV research, and one possible advancement the article mentioned was something like PV spray paint. You'd lay a wire grid down over the surface to be treated, then spray this nanomaterial over it and after it dried (and crystallized), presto, you have a PV panel. I'm sure that technology still has a long way to go but you could imagine the applications, like here, turning the entire surface of a car into a PV panel, with probably no weight or aerodynamic losses.


So, here's my (obvious) idea:

Hybrid postal and package delivery vehicles (UPS trucks, etc). They spend tons of time idling and hybrids turn the engine off. They spend tons of time accelerating and decellerating at low speeds, and hybrids are more efficient in those situations.

And -- they have huge flat roofs, maybe 16 m^2. Perfect for solar cells. Aerodynamics aren't affected if the cells are perfectly flat, and they'd gather energy all day long for the most part.

It would seem like the UPS trucks might be able to double their mpg in those conditions -- why isn't this happening? It seems like a slam dunk!


I believe that UPS is already experimenting with a hydraulic regenerative braking system on their delivery vehicles. I don't think the cost/benefit/useful life of photovoltatics is impressive enough to interest their beancounters yet. Besides, UPS's big flat roofs are already being put to some use - they are largely translucent, allowing natural lighting to the cargo area.

Kathy, UK

should look at SperalSolar, whose new system will permit building & vehicle integrated solar.



^ you prolly mean



Not to rain on anyone's parade here, but consumer grade PV is tops 15% efficient at turning the 1 KW/m^2 solar irradiance (at solar high noon) into power. The prius has mebbe a couple square meters, so thats 200-300 watts tops at peak production. Take 10-20% of that power to store it in a battery. That's miniscule output compared to the electric engine requirements on the order of tens of kilowatts of power. It all helps, but at $4-$5/watt installed for PV, the payback is a Silly Long Time. Also, you're dragging around heavy panel, prolly negating any advantage you'd get in non-steady-state driving.


This is what I've been waiting for!!!! To me - generating and/or capturing energy close to where it will be used just makes sense. I can't be the only one who has walked under power lines listening to them "hiss" and wondered how much electricity gets lost in "transport" along those lines. When you think of all the cars that sit in the sun while the driver is at work, it begs the question, if they had PV car bodies, could they drive home on solar alone? I wouldn't be surprised if we see PV car bodies before we see Hydrogen cars become mainstream.

Lamar Johnson

Marshall is right. It's a small drop in the bucket.

On the plus side, as Marshall noted, every little bit helps. It also has a Gee Whiz factor; it breaks the ice on solar cell applications among people who have never seen them before; it certainly has geek appeal; but I doubt that the energy required to make the solar array and controller will be recovered in motive power at the wheels.


Even if current thin-film panels aren't worth hauling around for most drivers (and the less you drive, the better they'll be), this is likely to change.  Plastic PV cells appear likely to be enhanced with the multiple exciton quantum-dot collectors.  These cells will be inherently able to be molded to any surface shape.  If you combine this with 30% or greater efficiency and possibly a memory capability of rolling over front and rear windows to collect light falling on them while parked (rolling back up for travel), the possible fuel savings will be much greater.

At 175 inches long and 68 inches wide, a Prius has 7.85 square meters of area.  If each square meter receives 6 kWh/day and it's captured at 30% efficiency, that's 14 kWh/day; if the car uses 250 Wh/mile, that's sufficient energy to cover 56 miles a day on every sunny day.

The solar-powered commute is a very real possibility.  We'll probably see someone doing it within the next five years, and commercial sales a few years after that.

Jim White

Everyone thinks solar is so expensive, but solar PV is now becoming cost-competive with gasoline. Do the math. A gallon of gasoline has a raw energy content of 114,000 BTU's. This is equivalent to 33.4 kWh (114,000/3413 btu/kWh). But a car engine is only able to convert 20 to 25% of this energy to useful work, so there is only 6.7 to 8.4 kWh of useful energy in a gallon of gasoline. At $2.50/gallon, gasoline energy costs the equivalent of $0.37 to $0.29/kWh. This is about what grid-connected solar goes for and solar is only going to get cheaper.
Steve Lapp has shown us that someday soon we'll see plug-in hybrid cars with integrated solar that assist the car while it is driving; recharge the batteries while it is parked, and put power back on the grid when the batteries are charged. While PV may not provide all the car's energy, it will help us eek out more miles per gallon.

Joseph Willemssen

According to my calculations, using Kingston, ON, as the location - given its insolation values throughout the year, and assuming 15,000 miles per year driven, driven an equal distance on each day, one would save about 12 gallons (about $30 worth) of gasoline with the 2 square meter, 10% efficient numbers Mr. Lapp provided. That's about a 4% efficiency improvement.

For comparison, improving a 20 MPG vehicle by 1 MPG will save 36 gallons of gasoline over the same distance - 3 times as much.

Even if one achieved 30% efficiency and had 7.85 square meters of PV surface, it would save 143 gallons over 15,000 miles - which is about 12 times as much as the project Mr. Lapp is describing. For comparison, upping a 15 MPG vehicle to 18 MPG will save 167 gallons.

Hybrids currently sold generally increase MPG ratings by about 50%, so say one hybridized a 15 MPG vehicle, it would probably get around 22.5 MPG -- saving 333 gallons per year.

These "solar Prius" experiments are interesting, but one could have far greater effect with much simpler solutions. For example, tires which are underinflated by 20 percent reduce fuel economy by 10 percent. Surveys have shown that about 30% of people have tires that are at least that underinflated.

So, if you think about a national fleet in the US of 180 million vehicles getting about 20 MPG on average, properly inflating tires would up average MPG by .62 MPG, saving the average vehicle 18 gallons per year.

Joseph Willemssen

Jim White, I think it's easier to go from the cost per mile of gasoline to see how much electricity would need to cost per kWh at the same efficiency.

At 55 MPG and $2.50/gallon, it costs 5 cents per mile for gasoline.

Given 0.262 kWh/mile efficiency of a Prius in all-electric mode, it means that any electricity that costs less than 17 cents per kWh is more cost-effective than gasoline at current prices. Pretty much any grid power in the US is cheaper than that.

A 123 watt Sharp panel (which is about 1 m^2 in size) costs about $600 from Real Goods. Assume 30 year useful life and average insolation values for Kingston, ON, and it can produce electricity at 13.2 cents per kWh over those 30 years.

The problem with the economics at this point is more about battery costs than anything. Makers of the Tango EV estimate that their 25 lead acid battery array costs between 7 and 15 cents per mile. Someone familiar with the plug-in Prius project could probably explain what that translates into costs for the extra battery array in a plug-in hybrid.


Mr. Willemssen, your comparisons would be more apt if you used the same vehicle, pre- and post-solar, for both.  Comparing a solarized Prius to a Durango is just nonsense.

Joseph Willemssen

"Comparing a solarized Prius to a Durango is just nonsense."

I'm not. I'm comparing gross fuel savings.

Solarized 1st gen Prius - saves 12 gallons every 15,000 miles. If the goal is reducing fuel consumption, then there are much better ways to save 12 gallons of gasoline.


I think a solar application would make tons more sense if you install PV's on the roof of your garage, and use them to help recharge your hybrid's batteries while it's parked inside.

That way, your garage bears the weight, not your vehicle.


Solar on the roof makes more sense right now.

But 10 years from now I can see you having both. Solar on the roof and PV paint on your car.

Why not have both. Personally I would like to see the Plug-in Prius concept be brought to market. I know Energy CS is doing it. But I would like to see some of the big dogs like Toyota and Ford get in on it.


An article which may be of interest to all is "Experimental Hybrid Cars Get Up TO 250 Mpg" at news.yahoo.com/s/ap/hybrid_tinkerers, which I came across on 8/14/2005. Another article of interest is "Brazil buys into flex-fuel cars" at MSNBC.com on 9/14/2004. The last article is "Muscle, Economy: Have It Both Ways With Ward's 10 Best Engines which I found at wardsauto.com/ar/auto_muscle_economy_ways on 7-4-2005.


I've been fascinated with the idea of adding some sort of pv component to my new gen Prius for a while. I live in SoCal, so we get a lot more sun than Ontario.

The ability to undock the PV would be idea, so that I'd be able to use it for my around town commuting where I'd be using the electric only mode as much as possible (I'd do that hack where you can go to electric only with the push of a button). But if I were doing a lot of freeway driving, say a 100 + mile trip, I'd want to stow it in the trunk to improve my aerodynamics.

I'm very interested in plug in hybrids as a whole, but I don't want to get my electricity from the grid, I'd prefer something sustainable like a solar panel. I'm glad to see that regular folks are working on the issues as proof-of-concept and that perhaps Toyota/Honda will take it as a cue to add these as options in the future. (The new Toyota concept vehicle does incorporate some interesting new developments such as this.)

Mikhail Capone


"I'm very interested in plug in hybrids as a whole, but I don't want to get my electricity from the grid, I'd prefer something sustainable like a solar panel."

Solar panels or a micro-wind turbine would be ideal, but have you considered buying your electricity from a green provider? I'm sure there are many to choose from in California.


good point on buying green electricity.

Jim Solar Stack

It's great to see the idea and potenial. It's not about the cost savings it's the idea and potenila.
MY solar is on my home and grid ties at the beat angle and area available in sunny AZ, I also drive a hybrid but mostly my bicycle to be real efficient.
On a vehicle it would be at the wrong angle, partly shaded and non aero dynamic . But it poves the point just like a solar racer in the solar challenge.
The point is solar is great, efficient vehicles are great but don't have to be tied together all the time to be used and learn from them.

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