> what would be the back up plan for in case of war, solar flare* or other natural disaster?

Does it need to be a natural disaster? Things like a widespread network failure, a major bank's systems going down for a while (happened not so long ago here), or even a country-wide blackout (we had one last week, triggered by a fault in a fault detection system mistakenly powering down a high-voltage transmission line, and for some yet to be determined reason that led to an instability which triggered other fault detection systems) can be a pain if you depend on an online system.

If an EMP truly destroys most of our electronic devices, money will be the least of your worries.

The world worked fine before electronics. Sure, the adjustment period would be harsh, but it’s silly to think civilization would end completely.

Your immediate need will be water, food, and security from it getting stolen. It is pretty much all three at once.

The cities will be awful places with death, destruction, and lawlessness.

The world had far fewer people before electronics, and they in part allowed for the agriculture to keep up.

Even assuming things like tractors etc. still work see how much yield local farmer will get once he can't get their fertilizers and pesticides shipped.

"harsh" is understatement, we're talking hundreds of millions dying from hunger within a year or two.

End completely, no. But the "civilized" way of exchanging cash for goods will be gone for quite a little bit. Long enough that we can figure out how to make more cash. Not that the existing one will be miraculously gone either.

There will always be a standard good for the basis of trade in proxy for government backed currency. The best choices will be durable (won't rot for a prolonged period of time), fungible (easy to convert back to digital cash), useful, and small to facilitate exchange at various magnitudes. Many choices will not fit cleanly and will all be less palatable. Notably, foreign physical currency, US stamps/bonds/checks, alcohol, or canned food would suffice. Ironically, a digital US currency should bolster alternative digital currencies.

Grains. The rulers of rural farmers group that establish grain production without industrial fertilizer supply becomes the new Habsburgs.

It's the Netherlands, we don't do backup plans for natural disasters anymore sadly.

Might be a great time to declare martial law and round people up in camps.

But solar flares that powerful are an actual serious threat, and have been recorded throughout history. It's just we have been so lucky to develop a heavy dependence on electricity in a quiet period.

The Carrington Event is the "most recent" in 1859 https://en.wikipedia.org/wiki/Carrington_Event

That would in the 2000s cause serious damage

>In June 2013, a joint venture from researchers at Lloyd's of London and Atmospheric and Environmental Research (AER) in the US used data from the Carrington Event to estimate the cost of a similar event in the present to the US alone at US$600 billion to $2.6 trillion (equivalent to $698 billion to $3.02 trillion in 2021[28]),[3] which, at the time, equated to roughly 3.6 to 15.5 per cent of annual GDP.

NOAA currently puts Carrington Events at a 1 in 500 year occurrence on Earth.

The point is that there won't be enough cash in circulation to run the economy for even a day.

That ship has sailed in NL, where only 20% of store payments are in cash (despite quasi-regular outages of the electronic payment networks).

How to get the cash when the ATM does not have power?