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From: John De Armond
Subject: Re: PV Panels On Vehicle Roof
Date: Sat, 08 May 2004 18:51:49 -0400
Message-ID: <>

On 8 May 2004 08:39:38 -0400, wrote:

>Ecnerwal  <LawrenceSMITH@SOuthernVERmont.NyET> wrote:
>> article in Home Power magazine some years back covered a person
>>who put a solar panel on the roof of their car and disconnected the
>>alternator, which resulted in considerable fuel economy improvement.
>Cool. I wonder how many watts the panel made.

Take such claims with a large grain of salt, as the math says otherwise.
Let's do a little.  The typical daylight load on an alternator of an older car
is about 25 amps (number is experience from many, many tests).  25 amps at 14
volts is 350 watts.  Someone more into solar electricity can do the analysis
but I don't think 350 watts' worth of solar panels could be fit to a car.

Assume that an alternator is about 50% efficient.  That means that 700 watts
of motive power would be required of the engine.  746 watts is a horsepower.
That's 0.94 hp.  A trivial amount of power.

To put this in perspective, my 94 Caprice requires 27 hp to maintain 60 mph on
level ground.  This car is quite streamlined and is sensitive to air flow
changes.  Rolling down the driver's window applies another 1.5hp load.  Older,
non-aero cars will require considerably more power.  In any event, adding or
subtracting 1 hp's worth of load is relatively trivial, particularly since the
car spends a significant amount of time NOT in steady state cruise.

I have little doubt that driver thinks he made a difference.  The reason is
that any self-directed test invariably contains experimental bias.  The tester
introduces bias favorable to the desired result whether he means to or not.
Most probably he drove slightly less aggressive after applying the solar
cells, wanting to give the conversion every benefit of the doubt.


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