Surely from the GPS position and time of day / month, you can get a very good approximation of the position the Sun
Yes, you can calculate it exactly.
The solar panels don't move around so you only need to know latitude and longitude at installation time.
You also need to know the direction of true (not magnetic) north.
A program called SunAlign was written in QBasic by David Williams:
David Williams
P.O. Box 48512
Long Branch P.O.
Toronto, Ontario. M8W 4Y6
Canada
This was converted to c by Marius Gundersen, January 25 2009.
The conversion to C was quite faithful to the structure of the original QBasic program, some variable names were changed.
I think there may have been some bugs in the C code I found.
I was intending converting Gunderson's code to run on the Arduino adding in comments derived from an explanation of the original QBasic program by David Williams, again with minimal alteration. unfortunately I got bogged down on other things.
If somebody wanted to collaborate on getting the thing running I would be interested to hear from them and might pick it up again.
There is enough info here to find the original code and explanation.