The schematic provided from wholesale solar calls for a #6 ground wire.
This seems a bit excessive as the remaining conductors are #10.
Fwiw: Grid tied system using 13 250 watt panels connected to micro inverters (enphase), 240 ac from roof to panel.

I looked in the NEC book and it's a bit confusing

Opinions please.

Note: some may reply with the one liner "speak with your local ahj".
What ever that is. What is a ahj and where do I find and ask questions?

"AHJ" - Authority having jurisdiction. AKA, your local inspector.

The grounding electrode and the grounding electrode conductor (or GEC) have nothing to do with the circuit or equipment grounding (the ground run with the circuit conductors).
Grounding electrodes are for lightning and other very high voltage surges. This is why it needs to be sized as it is.

NEC Article 690.46 may be the article that applies here...
The #6 AWG is used to connect (or bond) all of the mounting frames and panels to each other. 250.120(C) will allow a smaller conductor but only if it protected by a raceway.
As far as the installation instructions are concerned, they are enforceable by the code official and are required to followed by NEC 110.3(B).
Many inspectors are very helpful and actually do appreciate phone calls and questions from installers as it helps the inspection process proceed more smoothly...

Thanks, I wasn't able to find a recent NEC handbook for reading the 2001 provided a fair bit of info, in some cases it referred to NFPA (national fire protection) documentation.

I initially didn't realize that a solar panel array and mounting hardware couldact as a big lighting rod, so I now understand the requirement for #6 grounding wire.