That's me. I am mining only when my Tesla Battery is above 80%. I have a node-red automation that keeps all of this in check. Mining stops when battery reaches 60%. It basically runs all day from April to Oktober. My solar system now has a roi of 6-7 years if everything works out. 2 years in the experiment. I am planning to invest mining profits in other environment friendly projects. Here in Germany you only get 8 cents/kW from the state. Mining doubles that. Sometimes even triple. During the ETH run it was even 5 times more. Fun thing 90% of my overproduction still reaches the grid. My mining rig only uses 300 to 600 watts. Next I am planning to integrate awattar (dynamic price provider) to mine when the price of electricity is negative or close to zero. This happens sometimes during the night because of low demand and wind turbines going strong.

That is exactly the type of thing that miners that seek to capture wasted energy do.

What fraction of miners are doing this? Keep in mind this means they have to get pretty expensive equipment and leave it idle more than 80% of the time.

A large fraction because they get the energy almost for free. In Brazil they are using mining to compensate for the low viability in financial terms.