When society completely transitions to cash-less, what happens when the power goes down? End of the world?

Places with the wireless battery powered payment terminals will still work without power for a while. Lots of them don't even need network, they just sync up later.
But for the most part, if there is a significant power outage, things are screwed anyway.

Humans don't plan ahead for preventables and get angry if you diminish current QoL for long term safety. We know this. Earth getting a direct hit solar flare ends civilization. There's no divine safety guard rails, we'd just be fucked. Probably not extinction though. Just a lot of violence, extremism, and starvation getting to that point of very diminished stability again.
I like your use of "when" because it is fairly likely to happen one day. It already has, and Morse code had just been invented IIRC. Not that long ago, just wasn't a big deal then.
Did you hear the news about definitive proof of rogue black holes? Billions of them exist in the milky way it is thought now zipping around fucking shit up.

People deciding on this will have what they have, because it's other people obeying them first and foremost.
They also will have physical money when you won't. At worst it'll be pieces of gold or brilliants or whatever.
And you being left to rot in such a collapse is no problem. See how Russia's regime just threw out dozens of thousands lives of those they consider unimportant, to utilize in a war. Those were mostly uneducated men from poor and depressive areas, for whom the money for that contract was something enormous.
"The politicians" is not some rotated pool of people in reality, it's the same mafia layer. Most of them are of the same parts of the societies, there are no random people in power, at least not anymore. Not in the last 20 years, I think.
So, the answer to your question : then nothing. Your riots are inconsequential, they don't affect most of power, there are Pareto laws everywhere, so if actually important logistics and information flow don't stop, there may be riots for years without interruption, not changing anything. You might have read something like this about Iran, riots are a usual event for them, even despite rioters being sometimes murdered by security forces, sometimes even machine gunned. If something like that causes a problem for the elites, you'll see rioters being machine gunned in Europe.
The fact that we see this all means that somehow our side of the stakes has lost any leverage and it's all changing for how it's better for them. As simple as that. "Cashless society" is about that too - where all your money is controlled centrally and can be, well, momentarily taken from you, being just bytes of data on spinners somewhere, and all you do with your money is surveilled by default.
OK, this was alarmist, dramatic and soap-opera like. But I do think that, because with the previous steps of that path what I describe has already happened 100%.

Cash will always exist. Even though I pay cashless 99% of the time, there's always that little 1% when having a bit of cash on you is useful. It just means any cash on me will last a long time before I even get around to spend it.
And why would there be riots? Spain had zero riots, people were calm from what I've seen.
- Okay so. Say. There is 2 weeks without power, sudden and unannounced, unpredicted power outage. How will you get food and stuff?
  So if people can't get essential stuff, there would be fear, and with fear, riots are likely to happen. Doesn't matter how "civilized" or "developed" a country is, everyone has their breaking point.
  
  The existence of cash is probably the least of one's problems in that scenario. How is food going to be delivered to stores without working gas pumps? How will stores open their electronic doors or process payments without a cash register?
  If this is something you are worried about, store enough non-perishable (eg canned) food in your home so you don't starve in that scenario.
  
  In that scenario most of the food has gone bad anyway and is stuck in distribution centres as the shops can't send orders up through the supply chain.
  Also, without power most places couldn't take cash. Tills are computers that do all the maths so the 16 year old serving you doesn't have to they also track inventory going out.
  The cash that there is is stuck in banks because the banks have no way of knowing what money is yours as we haven't had bank books for like 20 years already.
  
  It’s called a natural disaster and we get along just fine. If the entire planet loses power, there’s nothing to be done, but even if an entire US state loses power, gas generators come online and trucks haul fuel in from long distances. It doesn’t take long for a grocery store or bank to open up with cash withdrawals again.
  
  This is silly. Absolute worst case, we go back to bartering for goods and services. There will never be a need to panic.
  
  As if money would be people's biggest concern of the power went out everywhere for two weeks.
  
  2 weeks without power
  How will you get food and stuff?
  You go there where it is, and ask. If they trust you enough to pay it later, ok. Otherwise you beg. And beg more.

I guess riots break out all around the world?
I feel like this idea that people are just going to riot and do mass violence is some right wing fear.
Most people, most of the time, are pretty social cooperative creatures.
- I disagree and think it depends on where you live

One huge condition will be: has your country invested enough in a super reliable infrastructure?
Spain wasn't bad in this regard (like most western European countries) but that recent event was hardly foreseeable. And they forgot to prepare for the "black start", which prolonged the problem.
In the future, the grids must become even more reliable and fault tolerant.
Some countries should better increase their invests by magnitudes.
- It was probably the best case scenario. This happening outside of winter or a war was pretty lucky.
  Now they get to learn and improve.
  
  Tbh, I heard it caused the UK government to actually revise it's plan for such a scenario as well.
- Microgrids with solar panels on every residential home is key

People will adjust. What happened on Portugal and Spain was caused by excessive centralization of the power grid, not by digitization. If somehow we can't keep the centralized grid running anymore, we will break it down, and bear the extra costs for that.
Also, the sequence of a catastrophe is almost never a riot. Where do people get the idea of riots? People just go and do the right thing.
Seriously, has none of the politicians ever thought about this?
The technicians did.
Where are the backups?
You mean generators? Lots of people have those.
Are we just going full “YOLO” on the reliance on the power grid?
I would understand this question if you lived in 1925, but by 2025 you should know the answer already. Are you so blind about everything that needs electricity that you think disaster would come from the lack of money?

solar power and batteries exist too you know
ik that stuff in Spain happened, but if there were more batteries it might notve
- Solar panels are enough for power during the day.
- ~~Cashless requires power all the way from PoS to wherever the servers live.~~
  Edit: see below
  
  Not 100% true: I know some places in Norway that have unreliable internet connectivity. They have terminals in the store that will save your purchase and wire it to the bank when connection is restored. Of course, this means you can over-draw your card, but I've never heard of that being a big issue in those small places.

look at sweden..hands out papers telling you to have cash for week...but doesnt accept cash anywhere. we all can learn from the dumb nations.
no cash means government controls who you can give money to. beggers,homeless ppl, panhandlers... are all doomed. if you cant get a phone, you cant have money. if you dont have a home or money you cant sign a phonecontract and so on...
cashless societies are nothing but a nasty techbro dream.
- beggers,homeless ppl, panhandlers… are all doomed
  In China, they have Wechat Wallet that people can give money to homeless people. Even homeless people have phones.
  cashless societies are nothing but a nasty techbro dream.
  It is nasty and dystopian, I agree. But its not really a fantasy anymore, its real, the dystopian future is on the horizon. Soon, it'd be too late to stop the dystopia.
  At first, mass surveillance cameras is only in China, but then even supposed "democracies" like the UK have millions of cameras. Then China became mostly cashless, that will also eventually happen to western countries.
  The dystopia is coming. You can't stop it.
  
  I visited Australia last year. There were surveillance cameras just about everywhere. Even on rural highways to catch speeders
  https://www.melbourne.vic.gov.au/safe-city-cameras
  
  heard that china pandler story too often by now. durinf sleep ppl tend to dream but reality will come they will wake up. maybe we are a cashless world by then...i wont stop it...but reality will. in a world of nations fighting each other digital money cant work.

What happens when an abused person has to escape a partner/parent who controls all the money? Where do they go, what food and board are they getting?
How do small traders set up garage sales and marketer stands, especially if they don't want to give cuts of their money to corporate giants Eftpos and Visa?
How do those with impulsively/memory issues (such as ADHD, dementia, and teenagers) manage the abstraction of their money, leading them to accidentally overspending/overdrafts?
How do you spot a stranger in need a bus fare home?
How do we support the street artists and buskers?
....I don't like the idea of cashless. My country already uses eftpos and visa as the norm (so ofc we all pay those overseas companies their fees). But while wide accepting of the card is good and useful, true cashless has issues of usability. It's not just 'something something government tracking spending'.
Vulnerable people fall through the gaps, and it means people make a lot more consumer transactions and a lot fewer personal ones.
- Cards destroyed so many lives but if you say it out loud people think you're a tin foil hat nutjob
  
  Cards themselves have been very useful. They're much lighter and harder to steal money than carrying hundreds in cash in your pockets.
  It's cashless that is a concern, not the existence of cards.
- The answer is: We're fucked.
  People were warned of mass surveillance, and here we are, cameras everywhere, over the entire world. Everything is tracked. Same thing will happen to paper money and coins.
  
  Cash is expensive for stores to manage, count, and sort. That's the actual reason they want it gone, not tracking. Sure, we're being tracked, but that's not the point. Thanks to our phones, our personal lives have already been completely disseminated.
  Cashless is about making things easier for businesses that struggle with handling cash. A cashless society acts like consuming goods from those businesses is the only reason money exists, and that's wrong.

Books has become e-books.
To some extent --- but have you been to a hip bookstore recently? They exist, and are very much alive.

In Spain credit cards still worked during the outage.
And the proposal for digital Euro already contemplate an offline mode for transactions.
As long as the power loss doesn't last days and batteries die out there would not be a problem with that.
And outage of days will bring so many problems that cashless society might be the less of them.
We can return to a primitive society to avoid dependence on electricity, but do we want that?
I think the best option is just people be prepared with food medicines and offline entertainment for a week in case of a big power loss.
- Spain here... How and what area are you referring to? Internet, Cell Phone Towers, Everything was down, no one was accepting credit cards in my neighborhood. The only thing they were accepting were IOU's (if you knew the store owner) and Euros.
  
  Center spain. Until 6pm we didn't had internet or electricity. But most TPV still worked.
  Here is an article explaining why: https://www.xataka.com/servicios/resolviendo-grandes-incognitas-apagon-que-algunos-datafonos-funcionaban-red-otros-no
  Basically there's two ways. The SAI of the supermarkets keep them going. Or they had the advanced models that accepted offline transactions.

What then?
Yeah it'll just be over.
Meaning, people would try to barter, which is really bad because it forces extremely bad trades, because it's so hard to establish a good value for things.
We 100% rely on consistently working electricity and network connectivity for digital currency to work.
Which is why we should never get 100% rid of cash, even if we transition to mostly cashless, people should keep an emergency stash of hard currency. The same way people should keep an emergency food and water supply, in case of power outages like the one in spain. We can secure our infrastructure against many things, but not 100% secure against everything. Keeping a few bottles of clean water, a little bit of essentially never perishing food and a little cash and a few candles really isn't too much to ask.
- For some reason it’s become commonplace to think that barter is what preceded and/or would replace cash if we ever lost cash.
  Anthropologist David Graeber has written a more compelling account of history with examples in a variety of societies showing that debt and ledgers are what came before cash and I’m thinking a system based off of them would probably be strong contender for a future without cash.

Yes, civilization ends if the power is out for weeks.
The politicians have thought of this, but "people can't pay for groceries anymore" isn't the top priority.
The situation will be like a war zone or a failed state. If the government can't restore power, its money will also be worthless, electronic or not.

Debt and ledgers.
Anthropologist David Graeber made a compelling case that this was the system in many different societies and places before cash. There’s nothing stopping us from doing it again. His book talks extensively about how each society handled repayment, the role of violence, interest, social hierarchies, etc.

Cash will change to a digital form or disappear, I don't agree with the people claiming it wont't.
Scandinavia is so close already, recieving cash is considered bothersome. No one uses it for anything anymore. Well.. Besides drugs.
Both electricity and the internet is critical infrastructure. Any downtime of either is really serious. It is however not rocket science to solve the biggest issues in regards to payments. As long as people can show their identity we can agree on tiny loans for stuff. Or just having the government bail out all verified purchases after the fact.
100$ per person isn't that much money. Any bigger purchases can be handled with invoices.
So I am more worried about heating in the winter and access to water and sanitation.
- This is bullshit. Everywhere I was in Scandinavia, you could buy with cash. If one shop didn't take cash, you just went across the street to the other that did. Even in small towns north of the Artic Circle.
  Even Scandinavia isn't stupid enough to become 100% cashless
  
  Oh, I must have explained wrong.
  Yes, everywhere is legally required to accept it still. But the usage is really low, and there has been talks of removing the requirements of accepting cash.

A cashless society means your overlords will have COMPLETE control of your lives. Of course the oligarchs will have secret avenues of cash for themselves.
- secret avenues of cash for themselves.
  Gold treasure.

Cigarettes and other things of value will become cash again.
- Luckily, there are Nuka-Cola bottle caps...

even if you have cash, what you gonna use it for when tax registers are electronic ? nobody is going to sell anything
- fun fact, businesses operate without power by using battery operated calculators and inventory pads.
  every minute a business isn't in operation is a cost to the business.
  I worked at Super Walmart decades ago. power went out for the whole town. the main HV lines collapsed after a tornado.
  mgmt marked all the ice cream down 80% and we were still checking customers out on generators.
  reduce risk, increase profits, mitigate losses. the only bad opportunity is the one you ignore.
  by god we sold almost every tub of ice cream in an hour.
  
  this is kinda hilarious lol okey understood

Come to eastern europe if you want to pay by cash
You can't steal/tax evade as easily with e-money

If society collapses, what good would physical cash be other than toilet paper?

Same applies to crypto-currencies like Bitcoin - as far as I understand they need constant power and global internet connection to function. Otherwise the network fractures into shards that have different views of payment history.
Maybe if the system breaks, it can be restored to an earlier state, and synchronized with islands which did not have the outage, once power is re-activated.
- That's a little bit easier because they'd "only" need a satellite dish and battery backup. It's less dependent on local infrastructure.
  
  Interesting did not think about it that way.
  that satellite dish will eat a lot of energy for bidirectional communications, starlink requires as much as a refrigerator, if I recall correctly
  maintaining that satellite network is quite energy intensive, but requires non local infrastructure

It won’t ever go 100% cashless. There’s too many coins and paper money floating around.
- The existing currency pool is not the reason. Paper money has a pretty short durable life, and coins don’t have enough value to operate society on. It’s actually a fairly big task for a government to maintain the currency supply.

Technically in countries with fiscal memory devices you can't buy anything from store that have no power today becaue of taxes.

Why do you think everything will become completely cash-less?
- I know its not the same thing, but we've completely done away with the barter system. Way back in the days of yore people would be swapping things for things, and then money came along and they were like "whaaaat you want to take my stuff in exchange for a few little discs?" And now it's totally normal.
  
  We have not completely done away with the barter system. It is just less visible.
  I worked on a farm for a while that grew vegetables, and bartered some of those vegetables for other products from neighboring businesses. We got a pallet of apples in exchange for squashes from a local orchard, and seconds frozen pizzas in exchange for veg for pizza toppings from a local restaurant. We even bartered labor. There was someone who came in every week and spent a few hours cleaning and organizing the tomato cooler in exchange for some tomatoes that were on their way out.
  In each case it was mutually beneficial for all parties to exchange in kind without using cash.
  So while bartering is not common, it still exists.
- The less people use physical cash, the less it gets produced. Eventually governments/mints will say “hey, you aren’t using this much anymore, so we’ll stop recognising it”, or something along those lines.
  
  That will never happen. Cash is useful for laundering money, and governments are full of people working for criminals that need to launder money.

Unironically, yes. If the "internet" were to disappear tomorrow civilized society would no longer exist. Too much had been built on an unstable jenga tower of technology. We put a lot of faith in the status quo existing despite the fact that little evidence suggests it will
- Especially with so much being built on top of AWS, even ICANN in some ways (although I'm sure that there are loads of backup DNS records to fall back on)

This happened in Ukraine when they were attacked with a cyberweapon (NotPetya) by Russia in 2017
If you want to know what happens when all of the computers (banks, bus pass scanners, grocery store cash registers, ATMs, etc) stop working, I highly recommend listening to this episode of Darknet Diaries
https://darknetdiaries.com/episode/54/
- I know nothing of or even care about IT stuff but I fucking love this podcast.

Fortunately this would never happen, for the reason you listed. Cash is king and always will be.

In that case we have to rely on Elons white power.
- The power isn't infinite, but it's enough.

What happens if the power goes out now? We're already going to be in a very degraded living situation with battery backups providing a minimum amount of connectivity.

If powers goes out for more than a couple of days, not having ~~cache~~ ~~casche~~ physical paper money dollars will be the least of your problems. If less, then it will not be a problem.
Our society runs on electricity.
- cache
  Off topic, but: Did you use voice to text on this comment? 😅
  
  Sometimes when I'm tired, I forget how your ridiculous language decided to mangle another set of sounds, so I type whatever I recon it might be phonetically, and hope my phone correctly corrects me.