> But how often is it cloudy and windless for weeks at a time?

'“Energy Droughts” in Wind and Solar Can Last Nearly a Week, Research Shows':

* https://www.pnnl.gov/news-media/energy-droughts-wind-and-sol...

See also:

* https://en.wikipedia.org/wiki/Dunkelflaute

I think it would be location-dependent (low risk that (e.g.) the UK would be windless for long stretches of time, especially off the coast).