Only when it’s dark, overcast, winter or really cold. Otherwise it’s mainly due to the extreme overcapacity required to handle distributed unreliable energy sources as well as an increasing fleet of electric cars, stressing every last kilometer of the grid. And windmills, a reliance on methane gas as gap-filling and a few other issues. (Sorry, I know snarking is frowned upon on HN - but we choose this collective delusion over the hellish, yet stable, Cherenkov light of nuclear)