An Occupancy Sensor saves energy by ensuring lights and fans are only turned on when rooms are occupied. The motion sensor detects motion in the room to turn lights on when when someone enters the room and turns them off when no-one remains.
CEILING VS WALL MOUNT SENSORS
Occupany motion sensors can either be wall or ceiling mounted. For small rooms and retrofit applications, an in-wall motion sensing switch is typically installed in place of a standard switch.
For larger spaces or where no wall switch is needed, a ceiling mount motion detector for occupancy may be used. Their 360 degree coverage and higher location enables them to cover a much larger area.
MOTION DETECTION TECHNOLOGY
Most sensors use PIR or Passive Infrared technology to detect the motion from someone entering or staying in the room. This style works well with a small to average room, small bathroom or closet where there aren't any obstructions.
Motion sensors using a combination of PIR and ultrasonic technology are used when rooms are larger or there are obstructions in the room like cubicle walls or shelving. This type of sensor can penetrate these obstacles within reason.
DIFFERENCE BETWEEN OCCUPANCY SENSOR AND VACANCY SENSOR
An occupancy sensor will turn lights on automatically when motion is detected and turn them off when no motion is sensed.
A vacancy detector will not turn lights on automatically - the user must do this when they enter the room. Instead it will turn lights off when the room is no longer occupied. In this case the user turns on the light only if they need it on and the vacancy sensor will turn it off if they forget.
Some detectors have the option to be used with both automatic and manual control.