If you have a heat pump, your bill is usually going to go up more then you would think in the winter, if it kicks into the AUX heat mode. If you set your thermostat down to 66 when you leave your house, then kick it up to 70 when you return, and your AUX heat comes on, you are using way more energy then if it is just the regular heat. When it's in the low 20's outside or below, a heat pump isn't really designed for that type of cold, so it kicks into the AUX heat more often then, as well.
If you've never paid attention to your thermostat and noticed when the AUX heat comes on, you may want to from now on. It sucks up way more energy and increases your electric bill tremendously.
More information about formatting options