One area of interest is the transition as the sun comes up/goes down. 
If it does this in a way to make the supply voltage change very slowly 
the pic may not get a good start.  Solutions might include using the 
pics built in brown-out settings or perhaps a supervisory chip. I would 
verify safe, good operation at startup especially to avoid having the 
system hang and not produce any power all day.
Good luck!

John Nall wrote:
>...it 
> will involve a PIC which will get no power except from a solar panel.  
> The voltage from the panel will go to a voltage regulator (probably a 
> 7805) to power the PIC.  The idea is that the PIC will start its work 
> when the sun comes up and raises the voltage from the panel high enough 
> to start things.  When the sun is going down, voltage from the panel 
> will continue to drop (and eventually cease altogether) and the PIC will 
> go off.  It is a non-critical application, and so long as I get 4 or 5 
> hours of operation on most days, that will be fine.
> 
> Now this seems too simple, so I figure I must be missing something.  
> Does anyone see any possible problems that would make it a non-starter?  
> (The project has to do with a trickle charger for my sailboat, which 
> normally has no access to shore power).
> 
> John

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