This is a good point that I had not considered, and I will add a few additional thoughts:

* In cold weather, solar heat gain can work in your favor as well. Much of the effect will depend on the orientation, shading, and properties of your windows, though. On the other hand, as another commenter pointed out, more sun in southern, cooling-dominated climate can also mean more, cheaper electricity.

* If you have a heat pump water heater, it will actually _cool_ your space significantly. The heat is transferred from your home to your water and mostly goes down the drain with it.

* At 65F (18.3C), most people I know would already be wearing a jumper/sweater. That's why I chose a lower target temperature for Berlin. The best source I could find[1] indicates that in November-December of 2022 (in the context of rising energy prices due to Russia's war with Ukraine), Germans actually kept their houses at 19.4C, on average.

* Maybe I'm moving the goalposts a bit, but I chose Berlin mostly because the numbers worked out conveniently. As someone who grew up in the American upper midwest, I wouldn't consider Berlin to be particularly cold. Phoenix, on the other hand, is the hottest city in the country and its summers are some of the hottest in the world. In general, the hottest cities are still closer to what we'd consider room temperature than the coldest are.

[1] https://www.cleanenergywire.org/news/80-percent-german-house... (original report is on German)

There’s some element of comfort vs necessity here, I think… really, people could be keeping their houses at, like… 55F and they’d be totally fine. They just need to get acclimated to it.

On the other hand, depending on the humidity, heats over like 85F start becoming a health risk for some activities.