Well, I don't know. Let's think about how you would turn off solar panels. I remember hearing about solar panels (~10 years ago?) that would actually break under very-sunny-but-zero-load conditions, so I did some research using queries like 'solar panels without load' and am led to think that this isn't usually a huge problem (with regular/modern panels?). But physically speaking, unless you cover the solar panels with a blanket, the amount of incoming sunlight won't decrease, and when you aren't turning that into electric power, it'll turn into some more heat.

If your solar panels are on an open circuit with no way to generate electric power, I guess the amount of heat produced in the panels would be roughly equivalent to the heat produced by other dark objects -- which may or may not be too hot. If you keep turning the sunlight into power to route some of that energy into the ground, that may be a lot if you're doing that in a single spot, but I don't think it would be impossible to come up with a workable grounding system.

That said, I'd be very interested to hear how a large solar farm like this turns off their panels.

We routinely dump lightning bolts right into the ground and they go well beyond multiple megawatts. They're very brief, but if lightning took ten seconds to strike, it would still provide several dozen megawatts of power. How quickly would we be melting the ground at the multi-MW scale? How many lightning bolts would it take before the ground around my house melted and flowed downhill?

The value of the machine being on is precisely that you avoid the costs of turning it off and then turning it back on. Those can be considerable, depending on what machine you're talking about.