It quickly clears excess moisture and then shuts off to save electricity. If the moisture level is not elevated, the motion sensor will switch on the light and the fan for only as long as you are in the bathroom. The combination of the two sensors provides effective control. A model with a motion sensor and night light is a great plus, especially with children. The 7-watt night light uses very little electricity and often provides adequate brightness to use the bathroom at night.
Some models also offer efficient fluorescent bulbs.
When shopping at your home center store for a bathroom vent fan, most models will look similar. Hidden internal features like sound-absorbing scrolls, rubber motor mounts, shape of the air chambers, etc. have a great impact on the sound level. Any models under 2 sones are considered quiet. Since the sound level is somewhat a function of cfm capacity, it is important to size the fan properly. Also a properly sized fan will vent the bathroom more effectively.