I got my first bills and I started seasons on my game in late fall I turned on my thermostat to warmer costs more and my bill was $26,000!! haha (note to self set therm at cooler cost less!!)
For me, I have yet to notice the increase. My Sims only have used the thermostat when it's a heatwave or freezing outside. So my Sims bills have remained in like 1K depending on where they live. Unless it hasn't been noticed by me yet...