i was hearing about a photovoltic paint... this might be good for a car...

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this would be awesome on days when you leave your car parked in the sun. come back to a fully re-charged car!!

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Figuring about $1000 for the panel, using it every day for 6 months @ 5 miles a day, it'd cost about 7 cents per mile over the life of the car, or roughly the same cost as gasoline at current prices. Added benefits include zero polution, not supporting the House of Saud, and the come-hither stares from hippie chicks.

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5 miles a day? Buy a bicycle!

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Fascinating... For European or American urban area style runabouts, this could be perfect with cheaper solar panels and a tailor designed vehicle.

Solar holds a lot of promise if only the costs could be driven down...

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I bet it takes more than one flimsy solar panel on the roof of a Prius to make it top heavy as this article claims. All the battery "ballast" in its "bilge" is one good reason why!

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Running the insolation numbers for Kingston, Ontario (where the owner of that Prius lives), and using the 10% efficient/2 m^2 panels, that solar mod would only save about 12 gallons of gasoline over 15,000 miles, or about a 4% reduction in gas consumption. And that's under optimum conditions.

There's far better ways to save a gallon of gas per month.

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"Figuring about $1000 for the panel, using it every day for 6 months @ 5 miles a day, it'd cost about 7 cents per mile over the life of the car, or roughly the same cost as gasoline at current prices."

Problem with that is that the sunlight from an average day in December is only going to produce enough electricity to move that vehicle 8/10 of a mile. The peak output would come in July, when there'd be enough average daily sunlight to move the vehicle about 4.4 miles. Over a year, the panel(s) would provide enough power to move the car in all-electric mode for 586 miles, or about 1.6 miles per day - not 5 miles per day.

Plus, you need to factor in battery, inverter, and other system costs amortized over their useful lives, as well as factoring in the added weight and drag, and the fact that you're not going to get optimum sunlight conditions (parking inside, in the shade, etc).

But even if you just focus on panel costs and assume $1,000 for 2 m^2 of panels, and knowing that gas on a 48 MPG vehicle at $2.50 comes to 5.2 cents per mile -- the payback period for the panels alone would be about 33 years or about 490,000 miles (ie, functionally never).

Only way to lower that payback period is to add more photovoltaic surface area (if possible) increase their conversion efficiency, or up the price of gasoline. There'd also be better output from the panels in a place like San Diego instead of Kingston, Ontario.

Again - there are numerous better ways to save gasoline than this approach.

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They have a Turbo Prius, it's come out to a couple local autocrosses.

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Joseph - I don't know exactly how you're doing your math, but I'm not sure I understand your figures. You certainly sound like you know what you're talking about more than I do, but the way I calculated it was really an average of 2.5 miles a day on pure electric (6 months @ 5m/d) which works out to be 912 miles a year, or 13,687 over 15 years (a conservative estimate of the life of the car and panels). Take $1000 and spread it over 13.7k miles and you end up with a figure of roughly $0.07 per mile.

The milage of the Prius itself (which I was calculating at more like 40mpg) only really comes into play when calculating the comparative price of gasoline, which at $2.50/gallon/40mpg works out to be about $0.06 per mile.

If this were installed in a car with worse gas milage - the Ford Escape hybrid for instance, the price per mile on electric would be lower than the price per mile on gas.

I'm not claiming to know more about this - these are all just "napkin numbers" but it seems to me that installing solar panels in hybrid cars seems to be an idea worth pursuing and shouldn't be written off just because the price of panels is high right now and the pay-off time on the technology is 15 years or more. At some point in the near future the efficiency of solar panels will increase significantly and the cost will drop, making the panels a wise investment either as third-party installs or by mfrs.

Oh, and I don't know how you came up with the 490,000 mile pay-off figure, but it seems you are being quoted as a source:

http://www.gizmodo.com/gadgets/vehicles/solarpowered-toyota-117930.php

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Hi, Bill. Good comments.

I have to admit I made a mistake in my initial calculation - I missed a metric conversion doing the calculations and understated the amount of miles one could power with solar electric from the car's roof array in one year. So I apologize for that error.

That being said, I cleaned up the spreadsheet and uploaded a copy to my server.

http://www.cogo.com/ab/solar_prius.xls

Variables are highlighted in yellow. Feel free to play with the variables and share the spreadsheet, if you like.

You can see that I used Mr. Lapp's figures for the solar array, your figures for the cost of that array, the EPA's fuel economy rating for a 2001 Prius, and the miles/kWh figure I've seen from some plug-in Prius advocates (albeit for a 2nd generation Prius).

The cost of the solar electricity per mile by definition equals the gasoline cost per mile at the end of the payback period, and obviously goes lower if the lifespan of the array is longer than the payback period.

This model also factors out the cost of batteries (which can actually be pretty expensive per mile when you amortize it) and other equipment to modify the vehicle to be plug-in solar. It also doesn't account for the additional weight and drag from the array.

Also keep in mind that the model assumes the car can be exposed to full sunlight every day of the year - so it would have to be parked or moving in an unshaded area any time during daylight hours. I assume the efficiency rating that Mr. Lapp used accounts for conversion losses from panel to batteries, as well as the fact that the solar panels don't track perpendicular to sunlight throughout the whole day. But that assumption may not be correct.

In any case, the power output figures for the solar array are almost certainly more optimistic than it will perform in real-world conditions.

On the second worksheet, I made a small chart to compare a base MPG rating with a target MPG rating to see how much fuel is saved by going from one to the other. This helps in giving context to the fuel savings achieved by the Solar Prius.

For example, using the data for the variables that you see preset on the first worksheet, the solar mod saves 20 gallons of gasoline per year. By comparison, going from 24 MPG to 25 MPG over 15,000 miles saves 25 gallons of gasoline. To get that kind of improvement from any given vehicle requires little effort or cost -- pumping up underinflated tires, going slower, driving a little less distance per year, higher occupancy per trip, etc.

Anyway, take a look at the spreadsheet and play around with the variables. I'd be interested to hear your feedback.

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Damn! Spreadsheets? I got pwnz0r3d!

Seriously though, that's good analysis. I see that your payback time is about 20 years, which is at the outside life of a solar panel. And of course the output decreases over time, so even your figures are optimistic. My napkin numbers are highly optimistic. But I'm a 'tank-half-full' kinda guy. Anyway, I think your spreadsheets are helpful in understanding the issue from a fiscal point of view and I appreciate it.

Care to add a spreadsheet about the declining value of the come-hither stares of hippie chicks?

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