The reason electricity is "free" on very sunny days currently is because there is little/no storage for it, and systems aren't built to be able to capture the value of that excess energy. However, in a future where nearly all energy is from renewable sources, storage will be essential. So it's highly likely at that point that that excess energy will just be used for pumped storage, or hydrogen generation, or whatever other storage systems will exist.

The wholesale electricity price on the Australian NEM, which covers 9 million dwellings goes negative basically every day, there's 11 million dwellings in the entire country.

You can see the price in the bottom graph here: https://opennem.org.au/energy/nem/?range=7d&interval=30m&vie...

> That's not how it would work, and folks are extrapolating the current situation, with very limited energy storage, to a future where storage will be essential.

Again, right now in Australia, grid-scale storage is extremely limited (which is excellently highlighted in your link in one of the tables under "loads", at only .9%). Power prices go negative because on the grid generation always needs to be matched with demand, and there basically isn't a place to "put" excess generation. I'm addition, while I'm not familiar with the specifics in Australia, many places have government subsidies that can make renewable generation profitable even when prices go negative.

Here are 2 articles, on Texas and Germany, explaining how and why prices go negative, and it's basically always due to a lack of storage and government subsidies:

https://www.rstreet.org/commentary/understanding-negative-pr...

https://www.cleanenergywire.org/factsheets/why-power-prices-...

Also electric cars are storage, which means they can cut the net price of electricity even lower than the minimum raw price.