There are many studies which showed how much overprovisioning you need to go to 100% renewable with different size grids. None of these are outrageous, but it's also nothing we currently need to think about. We can easily scale up renewable capacity to 70% without significant overprovisioning. So we really just need to build lots more renewables at the moment.

If you don't have renewables for a while, and transmission line availability flags, you import anhydrous ammonia from the tropics. So, you need only minimal over-provisioning and local storage.

But then you presumably need to build these extra power plants that can use that fuel?

A combined cycle powerplant is about 10% of the cost, per unit power output, of a nuclear power plant. So we could completely back up a renewable grid with CC plants and nuclear could still be much more expensive.

And, steam turbines cost more to keep working than CC turbines. But both are very mature, meaning costs won't change.

We have combined-cycle turbines already. More will be built wherever they will be needed.