An analysis of the energy and cost savings potential of occupancy sensors for commercial lighting systems

Occupancy sensors are promoted as one of the most energy-efficient and cost-effective lighting control technologies. Despite widespread promotion, however, minimal independent information exists about their comparative energy savings potential in different spaces, the coincidence of savings with peak demand, or the impact of sensor delay periods on energy savings. This lack of information leads to end-user uncertainty and specification risk, which has hampered market penetration. To garner information about energy and cost savings potential, 180 spaces representing five applications (offices, restrooms, break rooms, conference rooms and classrooms) were monitored for occupancy conditions and lighting operation for a two-week period between February and September 1997. Baseline occupant switching and occupancy patterns were established, and the effects of installing occupancy sensors with 5-, 10-, 15- and 20- minute timeout periods were modeled using data for 158 rooms. The results of this analysis support the following conclusions: Energy savings were significant for all room types ranging from 17-60 percent. This is due to the findings that most spaces are infrequently occupied (averaging 18 percent of the time) and that occupants in both public and private spaces are not diligent about turning off the lights, leaving lights on 11-48 percent of the time. The majority of energy use, and the predominant energy savings potential was found during normal business hours, not after hours or on weekends. These findings clarify the role of sensors vs. time-based controls to capture potential savings. Although the majority of energy use and waste occurs during the weekday, the sensor's largest contribution to savings is generally not coincident with a typical building's peak load period, or with typical electric utility peak billing periods. Sensor tuning for time-out periods from 5-20 minutes accounts for between 6-13 percent of the total savings potential. These findings have significant implications for understanding the trade-offs between savings and occupant complaints when commissioning sensors.