All south facing windows' blinds are angled to reflect or let sunlight through, depending on forecast and current temperature. I'm on the second floor in a suburban apartment complex. I've only needed to turn on the heater twice since winter ended, and it's been under 50 degrees at night for most days of the past month.
It's pretty convenient how the leaves spring before summer, blocking heat from the south side, then fall for winter, allowing heating of the floor and walls.
During the summer, I just put a box fan in a south facing window, blowing out, and open windows on the north side. I have a window AC unit but it's been in the garage for a few years now.