Needs storage*, what that storage is depends on other factors.
(* there's a "well technically" for just a grid, in that China makes enough aluminium they could build an actually useful global power grid with negligible resistance, but it doesn't matter in practice)
As it happens, I agree with one crucial part of your final paragraph — hydrogen is hard to store for any length of time (not sure you're right about comparatively low combustion energy but that doesn't matter, low energy density overall is accurate but I don't think matters).
I favour batteries for that because battery cars beat hydrogen cars, and the storage requirements for a power grid are smaller than the requirements for transport, so we can just use the big (and expanding) pile of existing factories to do this.
But hydrogen has other uses than power, and where it's an emergency extra storage system you don't necessarily need a huge efficiency. That said, because one of the main other uses of hydrogen is to make ammonia, I expect emergency backup power to be something which burns ammonia rather than hydrogen gas — not only is it much more stable and much easier to store, it's something you'd be stockpiling anyway because fertiliser isn't applied all year around anyway.
But you could do hydrogen, if you wanted. And some people probably will, because of this sort of thing.
> A square meter of PV provides a theoretical maximum of ~1KW at 100%. Even the experimental perovskite cells only get 45% of that. 450W/m^2. Whereas nuclear is measured in gigawatts per reactor with multiple reactors per plant.
This is completely irrelevant for countries that aren't tiny islands or independent cities.
Even then, and even with lower 20% efficient cells, and also adding in the capacity factor of 10% that's slightly worse than the current global average, Vatican City* has the capacity for 11.1 kW/capita: https://www.wolframalpha.com/input?i=0.5km%5E2+*+1kW%2Fm%5E2...
They are of course not going to tile their architecture in PV — there's a reason I wrote "that aren't … independent cities" — but this is a sense of scale.
(* Number 7 on the Wikipedia "List of countries and dependencies by population density": https://en.wikipedia.org/wiki/List_of_countries_and_dependen...)
> Then a storm hits. Far less sunlight.
That's what the storage is for
> Then something like hail hits. Damage to panels.
Panels are as strong as you want them to be for the weather you get locally. If you need bullet-proof (FSVO), you can put them behind a bullet-proof screen.
> Then there's the issue of security if someone wanted to cripple the grid.
The grid isn't the source; if you want to cripple a grid, doesn't matter if the source is nuclear, PV, coal, or hamster wheels.
> Nuclear is 24/7, rain or shine, wind or no, impervious to even hurricanes, and already has a robust security and logistics apparatus around it.
Really isn't 24/7, it's 70-80%: https://en.wikipedia.org/wiki/File:Worldwide_Nuclear_Power_C...
And mis-estimating the environmental risks is exactly what went wrong with Fukushima.
