My apartment has huge south-facing windows and is on an upper floor (an issue in the cooling season with an anemic wall a/c, as I posted about this summer), but recently they have helped keep the place comfortable without any need to turn my heat on.
I haven't needed heat or a/c for the past month and a half or so, which is nice. Even though we've had lows in the high 20s/low 30s and a few days where the highs were in the 30s, the temp in the apartment has stayed remarkably steady (right around 72F), never dropping below the high 60s and typically being warmer than that.
I'll certainly use the heat when I need it, but I'm enjoying the near-zero utility bills for the time being... I got a $30 electric bill for mid-September to mid-October, of which only $15 was usage (the rest being fees) - pretty much just the fridge, a fan, chargers, and a few LEDs, plus a few dishwasher cycles and a bit of stove usage. I'll be interested to see how much heat the place needs over the winter.