Dunkelflaute (dark and calm) is indeed a real problem for a renewable-only future. If you want to maintain current reliability statistics with solar/wind only,

Dunkelflaute (dark and calm) is indeed a real problem for a renewable-only future. If you want to maintain current reliability statistics with solar/wind only, you need to overbuild both storage and generation to such an extreme level that it might not be cost competitive in the end. You'll end up building your entire grid for the 100 year dunkelflaute event even though 90%+ of it will be idle most of the time, or you'll keep a bunch of gas plants idling to pick up the load in that case. Really we need to crack better storage. Improvements in hydrogen electrolysis or direct air to fuel could do it, but there are non-trivial technology hurdles there as well. I'm holding out hope for deep sea geothermal ala Peter Watts' Behemoth. Perhaps we can exploit the temperature difference between geothermal vents and the surrounding water to get stable and green base-load power.