Base Load is about storage.
Nuclear - the energy is stored in the Uranium (other radioactive elements are available) but hard to stop/start. Base Load!
Hydro - stored behind the dam (and Solar ultimately)
Coal/Gas/Oil - it's in there. (and Solar ultimately)
Solar - well there are various schemes supplying energy round the clock - they mostly involve thermal storage, but there is one French proposal that charged batteries, and the electrolyte was pumped out to storage and uncharged electrolyte pumped in. Storage!
Wind is solar ultimately, and harder to store unless you use it to charge electrolyte as above or pump water up a mountain, as with Dinorwic which serves mostly as Peak Lopping

The best solution for the planet is for us to use less, or gear our needs to the sun, any bright ideas that actually work?