If you are talking about something like a DC disconnect box on the side of your house for solar, it is usually a distance from the panels and electrically isolated so I would ground that to the normal house electrical ground if possible, or a direct ground as close as possible.

I think some electrical codes require a DC disconnect for solar at the side of the house.

I should qualify this by saying I am not a "certified" electrician and you should consult the National Electrical Code (NEC) and local electrical codes for grounding solar panels or other outside electrical equipment because it's a serious business.

There are two reasons I can see for grounding your panels, one is lightning protection and the other would be to keep you or workers from being shocked when using a high voltage solar system (over 48V), or maybe if a GTI fails in a strange way and sends AC to a outside wire somehow.

For lightning protection, I would run a wire as straight to the ground as possible from your panels or tracker. Lightning wants to go in a straight path to ground.

I have seen a solid bare copper wire used for this on a lot of systems, usually #6 ga. The idea is to get a path started for the lightning to follow instead of going down the other wire path to inside your house.

Some local electrical codes I have seen say to hammer in a copper rod 6 ft into the ground or if that can't be done to bury it sideways as deep as possible. You should look up the codes for your area.

Generally your AC breaker box is already properly grounded per code so I don't think you have to worry about that, and if you are running a plug in Grid Tie Inverter (GTI) then that has it's own 3 prong standard ground.

Some of the metal cases on GTIs have a nut to allow connection of a case ground, you could ground that to the normal electrical ground or an actual direct earth ground if the situation only allows that.

Solar panels are normally isolated to 600V, or 1000V in some cases, see the sticker on the back side. Which means the + and - wires are isolated from the metal frame by that much voltage.

Whatever wire size you chose for ground should be used throughout your system to ground each panel using a good connection to each frame. I would say #12 ga. is good and #6 is better for longer runs.

Hope this helps.