The effects of changing occupancy sensor time-out setting on energy savings, lamp cycling and maintenance costs

Most commercial lighting installations utilize occupancy sensors as an energy and cost-effective lighting control strategy. Despite their widespread use, minimal independent information exists about the effect of changing time-out settings on switching frequency, lamp life, lamp calendar life and energy and relamping costs. Consequently, this leads to confusion in the industry about the appropriate time-out setting for applications. To garner information about these effects, 180 spaces, representing five applications (offices, restrooms, break rooms, conference rooms and classrooms), were monitored for occupancy and lighting operation for a two-week period between February and September 1997. Baseline occupant switching behavior and occupancy patterns were established, and the effects of installing occupancy sensors with 5-, 10- 15- and 20-minute time-out periods were modeled using data for 158 rooms. Lamp cycling and time-on and time-off frequencies were examined to determine the tradeoffs between time-out setting and annual energy costs and savings, lamp life, and lamp calendar life for each application. The following conclusions were reached from this analysis: The percentage of time spaces are occupied varies by application and will impact potential savings from occupancy sensors. Despite increased relamping costs and decreased lamp life due to frequent switching, installing occupancy sensors saves lighting energy and reduces overall costs. For classrooms, private offices and restrooms, installing occupancy sensors provides a reasonable payback for the analyzed time-out settings. Because the cost differences are minimally different for classrooms, private offices and restrooms, the longest time-out setting should be considered to minimize false offs and assure occupant satisfaction. For break rooms, installing occupancy sensors provides a reasonable payback if the five-minute time-out setting is used; however, shorter time-out settings increase the likelihood of false offs and increases the possibility of user dissatisfaction. For conference rooms, installing occupancy sensors provides a reasonable payback if the 5-, 10- or 15-minute time-out settings are used.