You don't build a coal power plant speccing in how much variable renewable energy will be produced. You spec for over the max power - in the middle of summer, with all the air conditioners on, and then you throttle down from there. As a utility, you wouldn't depend on someone else's renewable energy if your job is ensure that an area gets power.

That said - the best time for renewable energies to provide power is when the sun is shining the hottest, which for solar based power, coincides nicely with the maximum power use.