Even on a cloudy day, there is some electricity produced. So that can be built at an excess capacity so that even on a cloudy day at least a known amount of power can be produced, and bitcoin mined using the excess when it is not a cloudy day and wind is producing as well.

But it seems in many areas the reliance on wind and solar has occurred before truly addressing the situation where there are days either or both of those will fail to produce even a respectful fraction of their capacity.

It will be interesting seeing how (or if) bitcoin fixes this.