To the point about temperature-dependent efficiencies, I've searched a bit and found this: https://www.researchgate.net/profile/Engin-Gedik/publication... (PDF warning)

They go as low as 10°C as the ambient temperature, and you can see that in the range they're testing, operation at lower temperature is always more efficient than at higher temperatures.

> we don't even build parks in areas that go above 20-25 degrees if there are no outside incentives

I'm not sure this is actually true.

Here in Germany, we have a lot of cloud cover, and PV parks still make sense economically. I'm pretty sure places with lower cloud coverage, lower land prices but higher temperatures still make PV parks viable.

I think we might need an physicist to explain it better than I can. That being said 25 degrees C is not exactly "optimal conditions" but rather standard test conditions (STC). The temperature coefficient of the solar cells is -0,something% (0,3 or 0,4 I think) per degree of deviation from the STC. So they increase their efficiency as you go below 25 degrees C and lose efficiency if you go above. I think the STC is 25C because that is where you'll typically get the best combination of temperature and sun hours with the least amount of snowfall, I could be wrong though.

> I didn't know of anywhere in the continental US that was always below 20-25 C.

I should've been both clearer on what I meant. Both because I should've written that it was "generally" don't go above 20-25 rather than "never", sorry about that. Tariffs are a key aspect of it though, because many countries where you do go above 25 during the summer have green tariffs, NGO bonuses and/or tax reductions. Germany, Holland or similar are good examples of where you'll get hotter weather than 25 during the summer, but you're also paid tariffs where their government programs will pay you X amount of money up til a certain MW production, at which point you're selling at market rates. In some areas you have the NGO's which are often "green funds" set up by large companies which grant financial bonuses to green energy initiatives. Sometimes for philanthropic reasons sometimes to balance their CO2 outputs.

Generally speaking it's a sort of silly calculations that you do to build the most financial viable plants. In Germany for instance the tariffs not only cut off at a certain MW production, they completely go away meaning that you'll actually lose the tariffs completely. You might think that was a reason to keep production low, but EU laws make it possible to split your physical solar plant into multiple different solar plants owned by different companies, which can then sell power individually and even sell to each other. Yes... But at least it gets the green energy plants build.