If you look at any one solar setup's output, yes it varies wildly every time a cloud goes by. If you're off-grid and responsible for your own power, this is a huge problem. You would need a fairly large solar array and some also fairly large batteries to keep you supplied with power. With a large, geographically distributed grid of different types of generation (Wind, Solar, Hydro, Nuclear, maybe a bit of fossil generation, and battery storage to sop up the excess), these problems become much more manageable.