Nobody needs that, but from my point of view batteries will be so cheap and abundant that we will likely get to having 2 weeks of storage just sitting around the grid or rolling on wheels.

People always underestimate where exponential cost decreases will take us. Current battery production grows by 10x in a mere 5 years. In a decade, the time it takes to build a nuclear power plant, we will grow our battery production by 100x. Not enough people take this seriously, or even know that the trend exists.

We practically don't see the sun in northern Europe during winter. And yes, the wind might not blow either.

I consider 2 weeks of supply a bare minimum.