[hacker234]

Yes, you must size your panels for the worst case, not the best case.

When it's overcast, the best position is pointing straight up, eg fixed at the correct angle for that latitude.

To put it another way, a tracker optimises your panel in sunny conditions, which is not worth doing. That's when you have a surplus of charge.

[gkop]

That's a good point. For a given deployment site, it might still be worth optimizing for sunny conditions though - depending on the number of sunny days and their distribution across the seasons, the availability/cost of storage from one sunny winter day to the following overcast day, and whether or not you care at all about peak output.