Pumped hydroelectric storage is a "big" problem in that the scale is large, but it's not particularly complicated. The solutions exist, and they're comfortably

Pumped hydroelectric storage is a "big" problem in that the scale is large, but it's not particularly complicated. The solutions exist, and they're comfortably boring, and trivially scalable and redundant. Moreover, we're still pretty far from the point where this even becomes an issue. We can accelerate the deployment of renewables a lot before this is a real constraint on anything.