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Ah, the dream! A car covered with solar panels, sitting in the parking lot all day while you are at work, silently charging the batteries by sunlight so you can drive home this evening using only an electric motor - the same way you got to work.  Complete independence from the corner gas station!  Completely free energy!  

But we don't see any such vehicles.  Why not? With gasoline at record high prices nationwide in 2008, this dream is becoming more and more prevalent.  (As a note of interest, Americans are unhappy and whining about $4.29 a gallon gasoline, while in Europe it has already been selling for [after appropriate conversion from Euros and Liters] over $10.00 (US) per gallon as of this writing [Jul, 2008].  So, really, Americans - What do you have to whine about?)

Sorry, world, but you are power hungry and you don't even know it!  The bottom line why there's no solar powered car for you to drive to and from work is this: your insatiable demand for power, speed, and comfort.  I won't even go into the power it takes to move that behemoth SUV you drive.  Let's talk simply about the energy to move a small car around.  Let's talk about the Toyota Echo.  And let's talk about the energy in terms we can apply to Solar photovoltaic panels.

First a word about electric cars. Electric cars already exist, and have since before the advent of the internal combustion engine. But this article is not about purely electric cars - it's about solar powered electric cars. You don't see any practical solar powered cars on the streets - this article it intended to help explain why. Today's (non solar powered) electric cars vary from the terrifically expensive "Tesla Roadster" with a range of 200 miles and capable of over 100 mph, but also sells for over $100,000 (US), to the RIVA, sold in Europe for about $15,000 (US) with a range of 50 miles and capable of only about 40 mph, to the ZAP Zebra, $12,000 (US) with a range of only 25 miles, and a speed of about 30-40 mph. All these non-solar powered vehicles must be plugged into an electrical source, typically the commercial electric grid, to recharge.

What you do occasionally see (example, below) are expensive, light-weight, experimental solar powered vehicles for competitions in the quest for a solar breakthrough! That's all! You'd be hard-pressed to build or buy one of these vehicles for yourself.

You will NEVER drive one of these to and from work!

But you may drive something like this:

This is the "Stella" (Latin for "Star") advertised as the "world's first solar-powered family car". It was built by engineering students of the Technische Universiteit Eindhoven, Eindhoven, Netherlands

The story can be found here: Stella Press Release


You can build your own (non-solar powered) electric car - buy simply getting any reasonably small car, removing the internal combustion engine, and installing an electric motor, batteries, and controller! There are actually "kits" available to do this. You will encounter engineering difficulties, but it is not rocket science so these difficulties can be overcome. You will end up with an electric car that will have the typical range and speed - about 50 miles, and 30-60 mph.

There is no doubt electric cars will be all the rage in the next few years due to the crumbling of the cheap gas market. Thus, you will soon see new claims of solar power, indeed, Toyota announced recently they will produce a few Toyota Prius models (in 2009?) with solar panels on the vehicle. Lest you think the "solar car" is right around the corner, think again - solar panels on the Prius will do no more than add a small gain to the electric range, or at least slightly offset the electricity used by the radio and air conditioner blower motor. A Prius with solar panels is NOT a solar powered car. Adding a few solar panels to a hybrid vehicle is futile and will only serve to make the owner's wallet significantly lighter.

OK, back to the matter at hand....

The Toyota Echo is a terrific small auto, perfect for daily commuting. But most Americans did not know this, opting for a gas-guzzling behemoth SUV instead. (With today's gas prices it's too bad Toyota ceased production on the Echo, in 2005, just before the price of gasoline instigated the much needed move away from SUVs).  The Echo has a small, 4 cylinder engine, and gets about 34 miles per gallon (MPG) in the city.  It was a comfortable, roomy little car, very economical to operate even at current gas prices.  If such a car could be "electrified" and charged only by the Sun, this would be nearly the perfect commuting vehicle.  For what it's worth, the Toyota Echo had a 1.5-liter engine with "variable valve timing" providing 108 horsepower.  This far less than that gas-guzzling SUV you probably still drive (because now you can't get rid of it!), but the point here is that the Toyota Echo was a fine, small, light-weight car (about 2050 pounds, or 908 kg), completely practical for the daily commute.  No one, and I mean no one can say that about any SUV!  

Now, the idea here is to see what it would take to completely operate the Toyota Echo with batteries and solar panels to recharge the batteries.  In the discussion which follows, we are not going to get into the details of how much the weight of the vehicle will change as we remove the gasoline engine and transmission, and install an electric motor, batteries and controller.  We are going to assume the mass of the car does not change.  This way we can compare the energy the vehicle expends on a trip now, using gasoline, and calculate what we would need to provide the same energy with solar power.  In reality, we would have to do a serious engineering analysis, and make many modifications to keep the weight as low as possible and perform some tedious calculations on friction and energy losses and incorporate ways to recover energy such as regenerative braking.  But the goal here is not to actually determine the specifications of an electric conversion, for now, we will simply take the vehicle "as is".  This is known as a "back of the envelope" calculation.  Physicists and engineers do basic computations just to see if the answer is tolerable or that the answer indicates an idea has merit or not.  Sometimes literally the back of an envelope is used!  Truth be had, these calculations were done on a 3 x 5 post-it note!

Our Back of the Envelope Calculation

As mentioned earlier, we want to talk about energy in terms of solar power.  Typically one talks about automobiles using "horsepower" and electricity is typically discussed in "watts" both of which are measures of power, but not too many people understand how many watts are in horsepower and vice versa.  You never hear anyone saying "That baby has 261,100 Watts under the hood!" But you do hear: "That baby has 350 horses under the hood" and you have a fair idea of the power of that car compared to your experience.  One "horsepower" is the same as 746 Watts if you are curious.  Unfortunately, solar power is not referred to in horsepower, so we will talk completely in watts.  

The Toyota Echo runs on ordinary gasoline, has about a 12 gallon tank, but for practical purposes we will consider the tank to be 10 gallons and one usually will fill up with around 2 gallons left in the tank.  At 34 MPG, that makes the range 350 miles between fill ups.  408 miles if you want to stretch it to an empty tank.

One gallon of gasoline has 125,000 BTU (British Thermal Units, yet another measure of energy) which we need to convert to Watts.  But, 125,000 BTU is total energy content of the entire gallon, and the "Watt" is a measure of instantaneous power.  When measuring Watts over time, we have the equivalent total energy as a BTU so we introduce the "Watt-hours", or W-hrs for short.  If you turn on a 100 Watt light bulb, you are "burning 100 Watts" continuously.  If you turn off the light after 60 minutes, you have burned "100 Watt-hours." That's 100 Watts times time, in hours.   1000 W-hrs is the same as "1 kilowatt-hour" or 1 kW-hrs.

Since 1 BTU is the same as 0.293 W-hrs, or 0.000293 kW-hrs we can determine the number of kW-hrs in a gallon of gasoline by doing a simple conversion.  125,000 BTU x 0.000293 kW-hrs/BTU = 36.625 kW-hrs.  Therefore, one gallon of gasoline contains 36.625 kW-hrs of energy.

This means if you could convert gasoline directly to electricity without loss, you could run a 100 Watt light bulb for 366.25 hours, or 15.26 days.  

Now here is something we have to understand and accept.  As much as 65% of the energy in gasoline is lost in the generation of heat, and making the mechanical parts of the engine work (friction).   (If you don't think there is much friction in an internal combustion engine and transmission, remove the spark plugs and try cranking the engine by hand sometime!)   This means that, including other losses, you are only getting 12.82 kw-hrs of energy use out of every gallon of gasoline you burn! (That's 35% of the 36.625 kw-hrs available energy in a gallon of gasoline).  Eliminating the wasteful internal combustion engine eliminates this waste - heat and engine parts friction, so we are correct in our calculations to eliminate this energy loss from our computations. So for our conversion to solar power, we will use 12.82 kw-hrs per gallon of gasoline instead of the actual energy capacity in gasoline.

Since I mentioned the subject of "loses" above, let me expound on that a bit. Losses could be added to the computations to cover such things as aerodynamic drag, power for accessories (e.g. radio, lights, etc.), rolling resistance, and so on. But these losses are already factored into the average MPG, so in the calculations above on how much energy we need to replenish, in using the average MPG, we have already taken these loses into account providing, of course, these loss conditions in an internal combustion engine vehicle are essentially the same as the loses we will experience in our electric vehicle.

So, back to the issue at hand. Let's say you have a relatively long commute to work each day, say 20 miles.  And, due to traffic lights and general traffic slow downs, this trip takes you 30 minutes of stop and go.  In other words, you are not really using the engine's entire 108 horsepower, you are averaging less.  But, since our Toyota Echo averages 34 miles per gallon, this average already takes into account this stop-and-go driving, we can simply use that average in our problem and calculate that we have used 20 miles worth of gas, or 0.588 gallons for the 30 minute trip.  Thus we have used 0.588 x 12.82 kW-hrs = 7.54 kW-hrs of energy to get to work. (0.377 kw-hr/mile). 7.54 kW-hrs is not much huh?  Let's press on.  

We will now magically exchange our gasoline engine and transmission with an electric motor and batteries for the trip back home.  We want to put solar panels on our Toyota so that, while sitting in the sun all day, the solar panels recharge the batteries so we can drive home on battery power alone.  The question becomes: "Can the solar panels charge the batteries enough to make up for the 7.54 kW-hr we used getting to work?"

If you are having trouble at this point understanding what I've done here, let me clarify.  I calculated that, using gasoline, you used 7.54 kW-hrs energy to move the Toyota Echo (with you as the driver) from your home to work, a distance of 20 miles. (I eliminated the energy you wasted in conversion of gasoline to heat and friction). What you need to understand is that regardless of WHAT you had for power, be it gasoline engine, electricity, charcoal, nuclear, whatever, you would have used 7.54 kW-hrs doing this job, that is, moving the car 20 miles.  So, if you had already converted your Toyota Echo to electricity by replacing all the power drive with an electric motor and batteries, you would have used 7.54 kW-hrs getting to work.  What you need to do now is replace that energy you used so you can drive home.  Our exercise here is to see how much energy we can replace using solar power alone.

A typical solar panel is a series of mono-crystalline or polycrystalline silicon solar cells, supplying about 130 Watts when the sun is shining fully on the panel.  We will assume our solar panels will always face the sun, even though this is completely impractical in reality. Solar panels are roughly 18 inches wide by 36 inches long, so we are unlikely to get more than 5 panels on our car.  The whole thing would be quite unwieldy, and would add considerable weight to the vehicle, but this is just a thought exercise so it can be excused.

Five solar panels therefore, in bright, direct sun, supplies 650 watts, total.  Assuming you have 6* good hours of sunlight, this means you are producing 650 x 6 or 3900 W-Hrs, which is 3.9 kW-Hrs.  

Thus, after all day in the sun, you have generated 3.9 kW-hrs, with 5 solar panels, toward replacing the 7.54 kW-hrs you used getting to work.  You fell short.  How short? Well, using the energy you produced you could drive just under 10.4 miles and you would have used up all the energy you generated!  There you have it.  Five large solar panels, generating 650 Watts every hour is inadequate by 50% - (remember we needed to go 20 miles!).

At this point, I think it's necessary to point out that the above calculations do not take into account a number of other losses, and fact-of-life restrictions.  First, I have not taken into account the amount of energy produced by the solar cells lost in the charging process. (All the 3.9 kw-hrs produced by the solar panels is not converted to stored energy in the batteries, you will always get some percentage lost in the charging process.) As mentioned above, I have not taken into account other losses in the vehicle - braking, aerodynamic, accessories (running the radio and lights), etc., because all these ordinary losses included in the average miles per gallon estimate, and no further complexity in the computations is required to account for them.   I have not considered the actual energy production of the solar cells from the fact that the panels are not going to be pointed directly at the sun all day, and the power output falls off by the cosine of the angle when the panels are not directly pointed at the sun.  So the total power generated, suggested above, of 3.9 kw-hrs, is very optimistic.

Thus you see why there are no solar powered cars on the road today.  The state of solar power technology is such that solar panels only supply, at best, half (actually significantly less) of that needed to move a common, light weight automobile any appreciable distance.

If you are to see solar power work at all in a vehicle there will need to be a giant paradigm shift in the transportation world. Large has to go.  Heavy has to go.  Fast has to go.  Air Conditioning has to go.  It is completely possible to build a solar powered vehicle today, and with engineering improvements and changes, a very practical vehicle can be built which would be useful for a short commute, albeit at low speeds.  But until the efficiency of solar photovoltaic cells greatly improves, an unlikely event, no vehicle the size and weight even of a small Toyota Echo, can be solar powered.  And, it should be pointed out that solar cells convert sunlight to electricity with an efficiency of only about 15%, (meaning only 15% of the solar energy falling on the cell is converted to electricity), and the maximum theoretical efficiency is only around 21%.  It is not possible to produce more efficient solar cells, with the current materials/technology though Martin Green in "Physica E: Low-dimensional Systems and Nanostructures", April 2002, reports efficiencies in crystalline silicon cells may reach the the theoretical limiting efficiency of 29% in the 2020 time frame! With cells that efficient, providing they were affordable, it would make it possible for small, light-weight vehicles to be completely solar powered.

OK, so you ask, since we are stuck right now with 15% efficient solar cells: "How many panels would I need, since 5 panels is not enough?" Since we are calculating anyway, it's simple enough:

We need to replace 7.54 kW-hrs, (7540 W-hrs), with panels producing 130 watts, charging for 6 hours of sunlight.  7540/(130 x 6) = 9.67 panels, but since you can't easily have 0.67 of a panel, round up to 10 panels.   Ten solar panels to generate 7.54 kW-hrs.  Ten panels would be 15 feet long by 6 feet wide and would dwarf our little Toyota! And remember, to fully produce the needed 7.54 kw-hrs, these panels need to point directly to the sun all day long! The sun rises in the East and sets in the West, and it will be most difficult to "turn" the whole car all day so the panels are always facing the sun!

As a final word, I was contacted by Mr. Carl Knox, a Storage Systems Engineer, who suggested something to me - rather obvious - which I had not considered. I assumed in my "mythical" electric vehicle, you would somehow carry the solar panels ON the car. That is to say, you'd mount them on the hood, on the trunk, and on the top of the passenger compartment. In this way, the panels would always be there, ready to produce electricity at all times, even while driving, so long as sunlight was falling reasonably directly on the panels. Thus, one would park the vehicle, and no further activity would be needed for the panels to generate electricity to recharge the batteries. This, of course, is not ideal in that the panels would almost always be at a less-than-optimum angle for power generation. Mr. Knox pointed out that the panels could be carried IN the car, rather than ON the car! Carrying out such an idea would carry the disadvantage in that it requires the panels to be removed, say, from the trunk, and set up - but at the same time the advantage is that the panels could be set up to more directly receive the sun's rays.

And a final, final word. Note that in the analysis above, we calculated what electricity we needed to replace the electricity we used driving our car only to work. realize that we will use the same amount of energy driving home (assuming level grade) - but now we don't have another 5-6 hours of sunlight to replenish the electricity we used driving back home! So our solar car must now be plugged in to our home commercial outlet to recharge - something we hoped to avoid! Hopefully you see the dilemma here. Even though we calculated above that it would take 10 solar panels would conceivably replenish all the juice we used getting to work, what we REALLY need is adequate capacity in our batteries AND adequate solar recharge capacity in our solar panels to get both to and from work! We need to be able to recharge all we used for the whole day so there would be capacity ready for our drive to work again the next day! Thus the comment above on the futility of Toyota adding solar panels to their Prius hybrid and by now you should understand the futility of trying to create a completely solar powered auto.

OK, I just can't seem to find the "final, final" word. Let me close with a comment on use of solar panels. Mono-crystalline and Polycrystalline solar cells are all wired in series to produce around 14-18 volts DC. (14 volt panels don't require regulation during charging of 12 volt batteries.) (In some home applications, expensive solar panels are now being wired to produce as much as 48 volts to reduce the resistance losses in the wiring to the regulator at the batteries.) It is very important that, in all cases of mono-crystalline and polycrystalline solar panels NONE of the individual cells in a solar panel be shaded. A shaded cell acts as a huge resistor in the series connection, and virtually all current in the panel ceases. Thus, in any solar panel installation using mono/polycrystalline solar cells, it is imperative for maximum current and charging the installation be such that no shade falls on any part of any panel at any time throughout the day. Amorphous Silicon solar panels do not exhibit this problem. A part of an amorphous panel can be shaded and the panel will continue to produce significant current, albeit reduced in proportion of the shaded area. The downside of amorphous panels, however, is they only convert about 5-8% of sunlight to electrical current - compared to 15-18% for high quality mono/polycrystalline.

Now, as you watch new hybrids, and new "plug-in" electric cars begin to hit the market, you are better prepared to understand why these cars will not, indeed, cannot be completely solar powered.


"Hat's Off!" Section - Stories of those who are at least pursuing the dream!


* Note on 6 hours of sunlight. In reality, solar calculations are typically done using less than 5 hours per day of usable sunlight.  In the winter time, in the Northern Hemisphere, there will be as little as 4.5 hours of usable sunlight, and in the summer, the number is more like 9.  Solar calculations typically take the worst case, which is the winter scenario.

Bill Welker,
August, 2005
Last updated Nov, 2015