> you can just turn solar off

Somebody has to go and turn it off, and having this person available overwhelms all of your operational costs.

Or alternatively, you need the infrastructure to do it automatically, what is currently expensive. (But there aren't intrinsic reasons for that being expensive, it's probably due to lack of scale.)

If it's just slightly negative, or just rarely so, it's not worth it.

Basically every home in Australia, and certainly every home and business solar setup, has a smart meter that is grid connected and can be remotely shut down when needed. Or they even just limit the amount that can feedback to the grid if required I.e you’re making 8kw of solar, but it will only let you feed in 2 if the system determines that.

There is not “person” turning things on and off.

Since this year, you're also forced to have the remote kill switch on the new solar installations at home.

Commercial solar fields are entirely automated, nobody is going to the site to throw a disconnect switch lol. For sites without hardwire internet, there’s 4G or satellite connectivity. A 4G cell comm module is a few hundred dollars. Adding in remote operations control is probably a tiny fraction of a percent of a solar field project.

Also, turning off solar (known as curtailing) is just dumb. It's throwing away "free" electricity. The UK and Ireland is doing the same by turning off their wind turbines every now and then, and it's frustrating.

By making the price go negative, you are creating the market incentives for someone to do something about it: households will invest in BES systems to suck up all that free electricity to use during peak times, and some industrious entrepreneurs might even be convinced to do it on a very large scale to start arbitraging on the price fluctuations.

You don't even need the price to go negative to have a BESS buffer make financial sense.