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