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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.



Dud Mytty

What sun is shining on your planet?

On planet earth, solar irradiance = 1 kwh/m2. According to the article, the pv panels will not charge while parked, the PV contributes only while moving. Therefore, 6 Kwh per day is not realistic. More likely, a 1 hour commute will net you about 200 watts (1 kwh x 1 meter x 20% efficiency), or about the energy required to move you 1 km. Of course, considering that you are (hopefully) driving at 60 km/h or more, your net increase is 1/60 - or about 1 mpg.

Further consideration...who commutes to and from work at solar noon when the sun is strongest? Expect about 10% of the pv output while the car is actually commuting (ie 7 am sun is much less powerful that noon sun). So your net increase is about 10% of 1/60...or about 0.1 mpg.

So what if you enhance the system to charge a battery pack while parked? Well...how many vehicles are parked all day in the sun - ie not in a parking garage or shaded by the sun?

Conclusion...Put the panel on a building where it's all day in the sun and it's not so complex as automotive engineering! Spend the money on a fixed system that is more logical.

Also, it is very necessary to be educated or we all look silly...and being wrong once lets all the naysayers reject other more logical solutions. The information to critically analyze this article should be known to all, or googled in about 1 minute...


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this whole thing is a good idea and i really didn;t read through proporaly so if i am stating something that has been mentioned well then sorry but andy way
ok so solar powered car is a great idea but if you take it up to the next level u could make a solar powered car WITH a rechargeable battery so when its not sunny you can still move incase the sun goes away in the middle pf the day!!



loai qdairat

Hi Sir,
Hope that you are doing more than fine .
well im asking for Panasonic Prismatic NiMH , it consist of 6 cells , it is 7.2 volt , 6.5 Ah , it is electro-thermal modeling , it is used for hybrid cars .
can you help us in this , and where we can find this part .
please advice .
best regards,


i got patents for wind turbine electrical genarator and exhaust gas turbine eletrical genator for hybrid cars ,with novelty.if you need contact me


i h8 u all

nice nice

titili wink




Great article. Many (not all) dumb comments. We need to get off oil. All efforts in that direction will speed the innovations needed to make it happen.

Think long range.
Remember, they laughed at the Wright Bros - imagine only 100+ feet on their first "flight" - how efficient was that???


as with steve lapp's project prius,hasnt anybody thought of that ,as well as an air compressor/generator
system working from the incomming air through the radiator grill of the moving car, even for a non 'green' car (Take out the heavy engine/gearbox put in four 12volt batteries and solarpannel roof then the
air turbine/generator,)granted high initial cost but how many MPG's were you wanting!

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