To estimate how effective occupancy sensors will be in a particular application, monitor lighting use patterns and occupancy patterns simultaneously. Use that information to determine the number of hours that lamp operation will be reduced and then calculate the energy savings.
There are a variety of ways to determine lighting usage patterns—some simple, some sophisticated. Among the simplest methods are interviewing custodial and security personnel about their observations or simply observing and recording whether lights in different parts of your facility are left on at night or in infrequently used rooms. Another simple method is to review the settings on lighting timers or building automation systems where these are in use. A more sophisticated approach employs datalogging: Battery-powered devices that log lighting hours are available for around $100 and can be used to count lighting hours, record the time and duration of use, and, in some models, correlate the data with sensed occupancy. You can place dataloggers inconspicuously in rooms and retrieve the data for later analysis. If one goal is to reduce peak demand, the most sophisticated loggers are more useful because they report when lights are on during the peak demand period.
You can determine occupancy patterns by using school and work schedules to determine when classrooms, lecture halls, and offices are likely to be in use. Occupancy hours (by students, employees, and cleaning crews) are generally well-defined for open-plan offices, lunchrooms, restrooms, and the corridors that serve them.
A commercial facilities study performed by one West Coast utility found that measured operating hours in halls and lobbies were 50 to 72 percent greater than estimates based on operating schedules; in private areas and conference rooms, operating hours were 29 to 46 percent lower than estimates. Guesses that are this far off could result in significant errors in estimated payback periods. Another thing to consider is seasonal variations in facility operation. Factoring these in can help you avoid incorrectly extrapolating one month's data to a full year.
Occupancy sensors sometimes yield smaller-than-expected savings because of building operations staff failing to adequately consider utility time-of-use rates and demand charges, improper product selection, unanticipated interactions with other building components, or improper installation. Users can maximize the performance and cost-effectiveness of sensor installations by considering these issues.
Measure cost savings. A large East Coast utility found that occupancy sensors installed under its rebate program yielded average reductions in energy and demand of about 30 percent. Any given installation can range widely from this average, however. Table 1 illustrates the typical range of savings in various types of spaces.
Table 1: Typical range of savings from occupancy sensors
Savings range by a factor of two or three in most applications, with the exception of open-plan offices. Actual savings may differ.
Evaluate cost-effectiveness. A reduction in energy consumption does not necessarily correlate to an equivalent reduction in cost. Sensor manufacturers tend to stress the energy savings from their products in their promotional activities and do not typically place as much emphasis on demand and dollar reductions, so be careful to evaluate savings projections in the context of your utility rate structures and building-use patterns. A promotional video from one sensor manufacturer states that the installation of occupancy sensors on a 10-kilowatt lighting circuit in a major New York City building reduced lighting energy consumption by 56 percent. However, because the reduction in peak lighting demand was only 20 percent, the dollar savings—taking into account the cost of power at the time of savings and the actual demand charges involved—were only about 38 percent (Figure 2).
Figure 2: Savings from a sensor installation in New York City
When demand changes are a significant portion of the total electrical bill—as in this case—the dollar savings from sensor installation may be considerably less than the energy savings if much of the energy savings occur during off-peak periods, when energy rates are lower and demand charges are not incurred.
Determine what kind of sensor you need. Ultrasonic sensors can detect small movements, but they are prone to false triggering. US sensors are best for covering large areas.
PIR sensors are resistant to false triggering, but they tend to have blind spots that get larger the farther you are from the sensor. These sensors are best for small spaces and detection ranges within 15 feet.
Hybrid or dual-technology sensors incorporate multiple sensor types in one device. The most common design combines PIR with US sensors, taking advantage of infrared's resistance to false triggering and ultrasonic's sensitivity.
Choose wall or ceiling mount. Wall-mounted sensors are best for smaller rooms such as offices, restrooms, and equipment rooms (such as printer or copier rooms) where people are only likely to be present for a short time after they walk by the sensor. In an open-plan office or where the lighting load is higher, mount the sensor in the ceiling. You can also find units that can be mounted in corners or on walls near the ceiling.
Ensure compatibility with other systems. Using occupancy sensors outside of their wattage ratings can damage or disable them, so make sure that the circuit wattage is appropriate. Many sensors are challenged by the larger surge demand of electronic ballasts, and some sensors will not last as long when controlling their rated wattage of electronic ballasts.
Install sensors carefully. Sensors are easy to spot, and people might be tempted to adjust, steal, or vandalize them, or they may just try to fool the sensors into perceiving a human presence when a space is unoccupied. To ensure continued energy savings, position the sensors carefully and train building occupants on their purpose. Proven effective techniques include:
- Involve building personnel in planning for the sensors.
- Train maintenance personnel and office occupants to keep sensors operational, rather than disconnecting them as problems occur.
- Position sensors so they only "see" the area intended to be observed—the most common cause of false triggering is incorrect positioning.
Alternatives to occupancy sensors. Sometimes, when occupancy sensors are not cost-effective, there are better options for turning off lights, including these:
- Custodial or security personnel can be required to switch off lights when they are not in use.
- Wiring lighting circuits into an existing energy management system may be cheaper than installing an occupancy sensor if lighting circuits are easily accessed and large areas can be turned off at one time.
- Individual areas can be controlled by timers that provide a defined "on" period.