The result is that most of today's lighting systems use more energy than necessary. Both types reduce lighting energy use, although one type may work more effectively in a particular application than the other. On-off lighting controls are best suited for applications where there are periods of time when no lighting is needed, such as private offices, conference rooms and classrooms. Although time clocks and timer switches have been used, occupancy sensors are the most common controls in use today. Most units have an adjustable time delay ranging from one to 60 minutes to prevent accidentally turning off the lights while the space is occupied but the unit senses no activity.
Occupancy sensors are particularly effective in areas that have no set pattern for occupancy.
Occupancy sensors cannot be used with high intensity discharge light sources that require several minutes to reach full brightness. Variable output controls are useful in areas that can make use of natural light. The control senses the total light available in the space, then varies the output of the lighting system to maintain a preset minimum level of illumination. The systems are practical for use with fluorescent and incandescent lamps only. Although automatic controls for lighting systems will reduce lighting energy use in nearly all buildings, actual savings will vary based on the hours of occupancy of specific areas within the building.