[londons_explore]

I suspect in the next 50 years electricity will end up globally transportable via undersea cables, like the internet does for data today.

At that point, it's always summertime somewhere and it's always daylight somewhere, and if prices were to fall to zero there is always someone who would like more heat for something.

Therefore I suspect zero-priced energy will stop existing.

[outop]

Isn't it possible that such a system will over-produce 99% of the year and that therefore, the marginal cost will almost always be $0?

'Take my energy and allow me to stop accelerating my flywheels which regulate production' seems more plausible than 'someone would always like more heat for something' (what?)

Or possibly 'take my energy and I'll cut off some of the people using spare energy to do low priority, low value computation for free'?

[londons_explore]

I think electricity use is far more elastic than you're imagining. Plenty of big users can turn up/down production and already do so based on prices. If wide price swings got more frequent, more stuff would get dynamic.

You can imagine home appliances having an 'eco' setting which runs the appliance like the washing or the dishwasher at the cheapest time in the next 12 hours.

Or the water heating systems which heat more water when prices are cheap.

Or heaters which switch between natural gas and heat pump based on price.

Or electric car chargers which charge during the cheapest hours.

(all of these already exist, but none are yet common).

Over the long term there is also plenty of elasticity. If electric heating is expensive, people will install gas/oil heaters when they renovate. If electricity is cheap, more people buy electric cars. With cheap electricity, maybe fewer people decide to add more insulation to their houses. Businesses don't upgrade energy inefficient equipment to be more energy efficient, etc.

Plenty of demand elasticity in both the short and long term. End result: As long as the market is unconstrained, prices won't hit zero more than say ~5% of the time.

[outop]

I disagree that this argument makes it less likely to have very low prices much of the time. I think it makes it more likely.

If peak to trough is a large gap, say 60% of peak, this tends to make it less likely that peak will be met by overproduction, since that would involve very large capital costs.

The picture you paint above would suggest a very small gap between peak and trough, say 2% of peak. This means that almost certainly there would be enough over capacity to more than meet peak demand. Therefore the total daily demand would be more than met by capacity, leading to some energy being thrown away. So at all times except the peak, the marginal cost would be zero.

You have given an accurate argument for why demand would be elastic at trough. But you haven't given any reason why overall demand would be very elastic.