My last bill was $350 for a 3BR house on a heat pump just to keep the temperature about 65!
Turns out my heat pump was broken. The inside unit ran but the outside unit did not. As a result the inside unit ran continuously. That seemed normal because everyone says that's what heat pumps do especially in very cold weather. It wasn't until I got the bill that I checked out the outside unit and found it out was damaged. So for at least a month the inside unit was running continuously and periodically activating the aux heat $trip$. I felt dumb for not checking out the outside unit earlier, but it's hidden away from daily life and I've never had a reason to check on it (until now!).
Rant time: I paid $75 to Southern Pride service to come do a check on the system. The guy said my thermostat was bad (it's NOT bad) because I installed it myself. He was baffled that I did not hook the E wire to anything--well, on this and other t-stats, that's the right thing to do. He offered to replace my programmable t-stat with a non-programmable one for $250 and to replace my $12 contactor for a mere $185, with no guarantees that either would fix anything. He did conclude my outside unit control board was OK, but as it turns out, that was the ONLY part that was actually broken.
I had another guy from G&S actually check my system. He fixed the control board, verified that I did wire up the t-stat correctly (he was actually familiar with the t-stat I used unlike the last guy), topped off the freon, and gave the rest of the system a thorough check.