I don't think the term "reliable" is the right term to use there. One should distinguish between reliability and availability. Renewables tend to be extremely r

I don't think the term "reliable" is the right term to use there. One should distinguish between reliability and availability. Renewables tend to be extremely reliable - why would a solar cell fail? - but not always available. Fossil or nuclear powerplants are quite available - they can run 24/7 on full power, but not always reliable. Especially nuclear power plants have a not so great reliability. They are often shut down completely and without warning for smaller non dangerous events. Also, in certain weather conditions, cooling of power plants is an issue so they have to be shut down. Also, for all fossil fuel plants, there is existing storage to compensate on a failing supply change (oil crisis) which have to be calculated in the costs. Of course, the availability of renewables is an issue that has to be dealt with in planning the grid. If they are cheap enough, overprovisioning helps a lot, having larger grid interconnects help - even if there is not always wind in Germany, there should be pretty always be wind in Germany or France as an example. We still can change the way existing plants produce electricity - adding pumps to hydroelectric plants to convert them into storage, store the gas used in biomass plants rather than constantly burning it. And of course we need to build up additional storage. But that is just at the beginning of the learning curve, as most grids currently can just absorb all renewables as they are produced.