But then you added the requirement of 90% uptime which is isn't how a grid works.

I'm referring to the uptime of the grid. Not an individual power source.

Assume we've successfully banned fossil fuels and nuclear, as is the goal of the green parties.

How much renewable production, and bess, does one need to achieve 90% grid uptime? Or 99% grid uptime?

If you want a balanced grid, you don't need to just build for the average day (in production and consumption), you need to build for the worst case in both production and consumption.

The worst case production in case for renewables, is close to zero for days (example). Meaning you need to size storage appropriatelly, in order to fairly compare to nuclear. And build sufficient production so that surplus is generated and able to be stored.

If we're fine with a blackout 10% of the time, I can see solar + bess beating nuclear, price wise. If the goal instead is a reliable grid, then not.

As an example: take Belgium. As a result of this same idea (solar/wind is cheap!) we ended up with both (1) higher greenhouse gas emissions and (2) costlier energy generation, as we now heavily rely on gas power generation (previously mainly russian, now mainly US LNG) to balance the grid. Previous winter we even had to use kerosene turbine generation to avoid a blackout.