power capacity coming from solar means that at peak, no new non-solar power plants are needed. it does not mean that at nadir everything is solar. legacy power plants continue working around the clock, it either goes to waste or it goes to things that run at night.
that aside, solar also includes storage of energy that comes from solar. you should read up on energy networks.
that aside, solar also includes storage of energy that comes from solar. you should read up on energy networks.