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!!)
I turned on the thermostat as soon as I put it in and have had it on the whole time, my bills went from around ~17k to around ~55k!! This isn't a problem considering my sims have a lot of extra money laying around (seriously, they've got around 1.5mil simoleons that I have NOTHING to do with)