I do not think that is a bug. There is not really an "off" option from what I remember. It is cooler or warmer, right? Well, cooler in the winter is still running. If you think about it, internal temperature control takes up a lot of power. I would pretty much never use them for off the grid play unless I had a huge production of power. Even then, it does not seem very true to theme when fireplaces and fans are an option.
Off the grid play is one of my favorite things with the IL, Eco, and Seasons combo. Speaking of, my favorite off the grid build which began before off the grid was a real thing in 4 is now the smallest type of tiny home. It works, but from my experience you definitely need more than one turbine or solar panel (but that is pretty realistic now, too, right?). I usually play with a family of six with that.
