1. How much "area under the curve" comes from _up to_ 90% of _peak capacity_. If it read "50% of annual consumption", I might be more inclined to ask your same question.
2. It doesn't necessarily follow that free fuel == cheaper power. I'm not at all familiar, but I imagine building, operating, and maintaining a solar plant could be expensive, perhaps even more expensive than a coal plant, for all I know.
The best way to measure this is from the CO2 emissions estimates. These fell by about 23% from 2018 to 2023[1], implying that renewables have displaced about one quarter of fossil fuel inputs. However, it should be noted that nobody really knows the true denominator of energy demand, because distributed small-scale solar production adds up to an unknown quantity.
In the spring and summer seasons California routinely hits over 100% of demand generated from renewables at some point in a given month. CAISO, which does not cover the entire state, says that in June about 47% came from renewables. Last June it was only 37%.
2. It doesn't necessarily follow that free fuel == cheaper power. I'm not at all familiar, but I imagine building, operating, and maintaining a solar plant could be expensive, perhaps even more expensive than a coal plant, for all I know.