Bathroom fans are great to have when you want to remove steam after taking a shower or bath. They will prevent mold, and save wear on your wallpaper or paint. Kitchen fans also provide a way to expel hot air or odors. However, in addition to costing money to run, they waste energy because they are removing the heated or cooled air that your furnace or air conditioner has provided. The longer the fan runs, the more of the warm or cool air from the house is being lost to the outdoors.
If you are buying a new exhaust fan for the bathroom, try to get one with a sensor, so that it will shut off automatically when the moisture is gone. If your fan doesn’t have a sensor, you might try setting the timer on your microwave for 15 or 20 minutes so that you’ll remember to turn if off. You could also do this with the kitchen fan.
Remember, also, that when your furnace is running, you may not need to run the bathroom exhaust fan as long. The dry air from the furnace will remove much of the moisture.