...can supply a maximum power of 1,728-megawatt and has a storage capacity of around 9.1 GWh.

Try harder, please.

Firstly, I would think the difference between production and demand on a single day is a better indicator than the difference between your daily production peak and your daily demand peak, which could be months away from the day production peaks.

Even that isn’t correct, though. Such a reserve buffer would be depleted after that worst-case day, so if the day after that day also has demand outstrip production, you’re in trouble.

If your goal is something not ridiculous, for example, storing storing the difference of production-consumption when production>consumption for use later when consumption>production, you would likely need very significantly less than 50,000+ of those.

For your average country even having 100 is difficult. Think of some flat place like Poland.

More than that, Germany isn't even the biggest producer/consumer of electricity... Imagine some place like China trying to ramp this up.

Clearly you only need a small proportion of grid power in storage to cope with peaks.

There are also multiple ways to smooth load - overcapacity, battery storage, pumped storage, load shedding, pricing. These are solved problems which just need a bit more work and investment, and many countries already use pumped storage extensively.

1. Small population.

2. Pretty tall mountains and sparsely populated mountains (see 1.)

3. Quite high yearly rainfall levels.

4. Very rich country.

5. Country with a very low level of corruption and from what I know, pretty efficient at building new infrastructure.

Is there an example of an "average" country having decent levels of pumped hydro storage at a level that's above 0.5% of the country's needs?

Scotland has already entirely met its needs with renewables this year. This can and will be expanded to other nations in time.

According to https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...

Although, at some point it could be easier to just build coastal lagoons and let the tide do the work.