It depends on where you are. I'd recommend using
PVWatts Calculator to get a fact-based estimate.
PV Watts is a great tool to get detailed, more precise estimates, but 4.5 hours per average day for coastal California is a pretty good estimate. For the sunny southwest, it's more like 5 hours per average day. Those are averaged over a year. You have to also add a typical 80% efficiency factor going from DC at the panels to AC to the grid.
Cottonwood Grove Annual Comparison shows the logged results of the 9.9 kW DC array at my house in Boulder. It has good southern exposure and is mounted on a south facing 8/12 (34˚) roof. I have had grid tied solar about as long as I have had a Tesla, and since I have owned a Tesla, I have produced far more electricity than my Teslas consume. The grid is a wonderful battery!
Let's look at the average hours of production per day for my Boulder array, averaged over 6 years.
14,182kWh/365.25days/*(9.9kW*80%) = 4.9 hours/day
I would use 250 Watts for a more typical production PV Panel today.
100*250W*4.5hr*80% = 90 kWh/day
All those panels at Hawthorne produce about 2 or 3 charges a day... If Tesla is really going to produce enough solar to feed the Supercharger network, they will need some large solar farms in other locations.