consider the implications for a post scarcity future

You gotta recharge your phone battery sometime though - and if electricity had a different cost for nighttime vs. daytime, you can bet that people would choose the day option whenever possible.

(I chose a mobile device here bc it doesn't need any "extra" battery or technology beyond what would already normally be at hand.)

Thats basically how its done in most of Europe. Price changes every 15 minutes and some smart system starting washing machines etc if a certain threshhold is reached.
Of course you can also get a hedged contract where you pay a fixed price and don't need to care about it, but you have the choice.
- Uh, in my part of Europe we don't have 15-minute changes, that would be a nightmare.
  You can have a contract where the day is split in 3 or 4 different rates, so that it's cheaper to run your washing machine at night for instance.
  
  I don’t get why that would be a nightmare. In my country the electricity prices change per hour for dynamic contracts (they just follow the energy market) and with normal usage it’s cheaper on average than fixed contracts, including those with peak and off-peak rates. For gas it’s a day price, again same as the energy market. For both electricity and gas the prices for the next calendar day are published in the afternoon (that’s how the energy market works). The companies charge a little extra per unit and a small fixed fee per month.
  Contracts with fixed rates (including nighttime and daytime rates) have to buy in advance, which means that unforeseen circumstances are included in the price and they also have to account for the fact that they might need to buy extra or sell off their excess based an actual usage.
  
  It's priced per hour, and fairly low slopes, I think. Haven't looked at actual smart grids, though. Basically you'll know that electricity will be cheap (or even negative net) the following night or day or that there will be certain very expensive peak times from 8-10 and 15-17 or so.
In places like Spain, there are different energy plans and some do include "Peak" and "Valley" price variances. Peaks are high demand, like when cooking dinner, "Valley" are the opposite.
You can adapt to these inefficies, sure, but doing so still takes more planning and effort (in this case in carefully timing one's phone charging, and in avoiding power using activities like that during non ideal times) than if there was no mismatch of availability and demand. It lessens the impact of the problem, but does not entirely remove it.
- it’s a fundamental inefficiency that must be worked around with additional effort and resources
  In the OP the use of the word "problem" rather than something like "challenge", and referring to the problem being the pricing structure (negative) makes it seem like we've switched topics slightly, but if you are just referring to the foundational inefficiency of energy distribution then yeah I agree it is definitely a challenge. However, that challenge need not be so overwhelming (even perhaps solely wrt pricing) that it negates the benefits of having that form of technology available altogether. e.g. if the power company itself, or each recipient building individually, had its own battery (if let's say those were cheap & sustainable) then that could work, without the users needing to care much. I forget which city but one example in Germany iirc pumps water up a mountain during the day, then at night or on a cloudy day that potential energy falling back down generates electricity again. So yes a "challenge" for sure but not necessarily an insurmountable one!:-)
  Also, there are "problems"/"challenges" wrt use of fossil fuels as well, which have implications for climate change, and therefore even purely from a profit perspective there's government laws & subsidies and public perception that can affect it, which could push the overall net towards being beneficial to store that energy for later.
- The answer to this is local energy storage. It could be at the home level, but doing it by neighborhood/industrial block would be better
  Then, you lessen the strain on the grid at large, and you also capitalize on the periods of low demand. This means less spot energy production and built in storage, making it easier to make the most of renewables while minimizing the need to fire up a natural gas plant to make up the difference
Most places in the US have peak and off peak hours with different pricing already. Certain smart thermostats can take advantage of this for running your AC and such.

Like turning them off... Which is fine. Turning off solar panels is literally built into the systems and can be automated

Sure, but you're not getting as much output from your panels as you could in total that way, making them less efficient overall. I'm not saying you can't run a power grid on this stuff, just that the adaptations to use them in a grid effectively have costs, and those costs are not exclusive to capitalism

Enter nuclear fusion… unlimited energy always and forever.

It's pretty easy to imagine fusion being great - but it's still just in our imaginations. No one has yet been able to build a working fusion power plant. There has been progress over the decades that people have tried, but its still a way to go yet. So although we can imagine that it could produce clean and plentiful energy, we just comparing sci-fi tech to current tech. The future reality might not be so great, and the current reality is that fusion power isn't possible at all.
To illustrate my point, lets imagine solar power from a 'theoretical' point of view, like fusion is described. Solar power uses no fuel; gets its power from sunlight. There is enough energy coming from the sun to meet the whole world's energy needs with just reality small amount of area. Solar power produces no waste biproducts... So if we just imagine the benefits of solar power, it sounds pretty much perfect. In in reality though, although solar is very good, it is still far from perfect. Construction, maintenance, and disposal of the panels are where the costs are. And so to compare to fusion, we'd need to know what it would take to build, maintain, and disposal of the fusion power plants. Currently we can't do it at all - so the costs are basically infinite. But even if our tech improves to the point where it is possible... it's hard to imagine it will be easy or cheap - especially because there will be radioactive waste. (Radioactive waste not from the fuel, but from the walls and shielding of the reactor, as it absorbs high-energy particles produced by the operation of the power plant.)
- Every previous adoption of technology has taken - what, 50 years? - between having the technology and being set up to make use of it. Gasoline did not immediately have car engines to put into, nor kerosine a whole city's worth of lamps set up to receive them, etc.
  Though at first, if fusion could power up the existing electrical grid then it could e.g. make electrical cars more efficient in the net/overall sense, even if vehicles operating directly on fusion power themselves would take many more years. So fusion really might be different than those that came before, if we are anticipating and more ready for it than previous technical advances?
  Though yeah, it will have its own challenges e.g. the radioactive wastes, so fusion would not begin to replace greener energy approaches such as solar, wind, and geothermal, only perhaps supplement them.
  
  radioactive wastes
  afaik, this isn't a thing for nuclear fusion. fission, to a very limited degree. yes, but fusion, no, not really.
  
  Not for the direct reaction itself but I thought there was something about spraying the container down or some such... I am probably entirely BSing here:-). In any case, whenever someone figures out a method to make it practical, then we'll see whatever downsides there may be to that:-P.
Just 20 more years of research. At least text was predicted 1990. And 2000. And 2010. And 2020. And last year.
I'm personally very excited about how it does seem to be finally making progress if slowly, but realistically, I'm less convinced that it'll be the solution to all our energy needs than many are. The physics of the process itself is very efficient, sure, but the kinds of machines needed to harness it are literally among the most expensive and complicated things built by humans, and they don't even produce net energy yet. Granted, the cost of such things should be reduced once they are industrial machinery and not exotic scientific instruments loaded with experiments, but I'd bet that the reactors themselves will still be incredibly expensive and complex (and therefore have expensive maintenance). This doesn't say good things about the actual cost of the resulting energy, even if the fuel is quite abundant. We could get abundant energy with a similarly high if not quite as much fuel efficiency with advanced fission reactors and fuel breeding, but the cost of those kinds of plants has been relatively prohibitive, and the costs of renewables has been falling. I could certainly see it possible for fusion to reach net energy, only to get used only on specialized roles or for base load power because solar panels end up being cheaper. In a sense this has already happened. It is theoretically possible, if not practically desirable, to use fusion energy in a power plant already, by detonating fusion explosives in a gigantic underground chamber full of water to heat it up, and harnessing the steam. Such ideas were considered during the cold war, but never developed, at least in part because it was calculated that they wouldn't be cost competitive compared to other power options.
- FYI https://www.theguardian.com/environment/2023/aug/06/us-scientists-achieve-net-energy-gain-second-time-fusion-reaction
  
  That's not really net energy gain from a practical standpoint. Technically yes, they get more energy than was present in their lasers, but those those lasers aren't created perfectly efficiently, and so the actual electricity needed to create them still is much higher than the energy output of the reaction
- I agree with you, but I don't think it means we stop the pursuit. It won't be viable or cheap enough in time to help in the transition off fossil fuels. If it does pay off the way some people think it may be a viable energy source for carbon sequestration to undo some of our stupidity though. I think it's worth that moonshot.
  
  Oh I wasn't suggesting we should stop the pursuit. I just think it won't be a magic bullet for solving our energy needs the way some proponents seem to suggest it will be.
not exactly but for all practical purposes might as well be
Solar power is just really inefficient nuclear fusion

Giant rubber bands.

You're welcome

It's definitely a problem with the grid, since too much supply is at least as big a problem as too much. Hopefully we'll get things like molten salt batteries so we can soak up this excess and decarbonise heavy industry.

Why couldn't the solar panels simply be turned off - is that not an easy solution to having too much intake?
- You're wasting energy then, and you also need to have some controller on each one to communicate with the grid. No country has a smart grid yet.
  
  Yeah but you said it was a "problem" - like I dunno, likely the excess energy start a fire or something? - whereas turning them off seems like it would reduce that to the system merely being less efficient than would otherwise be possible.
  Anyway, definitely some kind of energy storage battery seems naively to me like it would be the best solution, even if used in conjunction with several forms of energy production (solar, wind, geothermal, maybe biomaterials etc.).
  
  They do shut off ("curtail") renewable energy because it is a problem - excess power can destabilise the grid, causing brownouts and blackouts and also physically damage grid equipment like transformers and transmission lines over time.