I think most of the answer is right here in the v30 Introduction. The one piece of missing info is what interval is used for the calculation. (that would mean Solar Production / Total Usage for a short period of time)
In both sections, you can see your percent “powered by solar." This is an important metric that indicates how much of your usage has actually been covered by your solar production, rather than just being sent to the grid.
I did a quick experiment to figure out the update interval - It looks like that value updates in the same 1/2 second interval that the rest of the solar display does.
Initial Solar
5 seconds later, after starting my Tesla Charger
