Let me guess... you posted this with the Medium Android app, intending it to be in response to my post below, but the app has put it in the wrong place, where I won't see the alert from it (This is why I stopped using the stupid app).
Anyways, in answer to your question of figuring out what is needed in terms of solar panels to meet a given constant demand, the peak capability of the farm needs to be divided by a factor of between 5 and 6, regardless of the form of backup used, since what we are doing, is integrating the peak power and averaging it across time.
So in the case of Vietnam, where the known peak capability of solar power their grid can handle is 2GW, and they have 16.GW solar, if the farms were equipped with hydrogen backup, they would be able to supply a continuous hydrogen load of around 3GW between them.
On cost, the money-fuel tree article shows how to convert some (Or all) of the energy received to revenue which can be used to maintain and expand solar plant capacity at no additonal expense. None of this is technology that I have dreamed up, it is all standard stuff, the hardware is available to do all of these things from online vendors.
A hydrogen plant is a complex multidisciplinary system, which I and a number of other folk are in process of modeling and simulating using specialist systems modeling tools which have thousands of hours of PhD man-hours in each modeling primitive. This is with a view to eventually contributing to tools which will enable moderately skilled folks to knock up bespoke solar hydrogen systems for any community system, in any part of the world.
Meantime there are standard systems which can be just dropped into most situations, by heavily overestimating what is needed, they are easily obtained
