The availability of solar and say a coal fired plant are totally different numbers. Coal fired plants have reliability numbers for available output which is darned near one hundred percent of scheduled. Yes, there are failures from time to time, but the availability of a coal plant is really good and when they are making power, chances are it's going to be 100% of scheduled.
Photovoltaic solar, on the other hand, has variability approaching 50% of scheduled capacity ALL THE TIME. Lets say you have a forecasted 100MW solar capacity on line, you can only count on a fraction of that to be available and you need to have reserves to cover the variable fraction, even if you get what you forecasted. Wind is similar only it's worse, usually having about 35% (65% variable) capacity. It's hard to know where the sun will shine and how fast the wind will blow over the next 10 min, and you simply must as a grid operator KNOW how much power you are generating and how much you are using and they must balance or really bad things happen to the grid (think blackouts and equipment damage..)
So this reserve capacity needs to be up and spinning, ready for energy production when you put photovoltaic solar and wind into the mix. More reserve capacity than you would need w/o the renewables and their unreliable energy sources....