Lighting Occupancy Sensors

Occupancy sensors detect the presence or absence of people and turn lights on and off accordingly. Used properly, occupancy sensors can be a cost-effective tool for reducing the operating time and output of lighting systems, cutting energy consumption and—usually to a lesser extent—peak demand. They may reduce lighting energy consumption by 50 percent or more in some circumstances, but the savings could be much smaller, so it's important to carefully consider a wide variety of issues before installing an occupancy sensor in any specific location.

Occupancy sensors are used most effectively in spaces that are often unoccupied, including some offices, warehouses, storerooms, restrooms, loading docks, corridors, stairwells, office lounges, and conference rooms. Open-plan office spaces, where one or more people may be moving in and out throughout the course of the workday, are not good candidates for this technology. Occupancy sensors can also be used to meet codes and standards—including ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) Standard 90.1—which increasingly require some form of automatic lighting control for new construction and renovations.

What are the options?
this section

There are three types of signals that occupancy sensors use: Most detect heat (infrared radiation) or sense shifts in the frequency of reflected ultrasonic waves, or some combination of the two. Units that use acoustics are also available, but are far less common.

Passive infrared (PIR) sensors. These are the most common type of occupancy sensor. They are able to "see" heat emitted by occupants. Triggering occurs when a change in infrared levels is detected—for example, when a warm object moves into or out of view of one of the sensor's eyes. PIR sensors are very resistant to false triggering. Although some PIR sensors have an operating range of up to 35 feet in specific directions under ideal conditions, they are most reliable only within a 15-foot range because:

  • The blind spots between their wedge-shaped sensory patterns get wider with distance—the sensor is most sensitive to movement laterally across the field of view.
  • They are passive—they don't send out any signal—and depend on the intensity of the heat from the moving part of the subject, which drops with the square of the distance.

Ultrasonic (US) sensors. These are active: They emit a high-frequency sound (above 20,000 cycles per second, so that it's beyond human and animal audibility ranges) and listen for the reflected sound—a moving object will introduce a frequency change. Because they emit a signal instead of passively receiving, US sensors are able to cover larger areas than PIR sensors and are noticeably more sensitive (see Figure 1). US sensors, however, are more prone to false triggering. Air motion—caused by a person running by a doorway or a ventilator cycling on or off—can trigger a poorly located or maladjusted sensor. US sensors can also be triggered by curtains, shades, or blinds that move with air currents.

Figure 1: Sensor coverage diagram
Ultrasonic sensors can detect motion at any point within the contour lines. Infrared sensors see only in the wedge-shaped zones, and they don't generally see as far as ultrasonic units. The ranges are representative; actual sensors may be more or less sensitive.

Acoustic sensors. Acoustic sensors detect noise made by people and mechanical noise related to human activity, such as keyboard tapping, paper shuffling, and photocopying. Unfortunately, these sensors also respond to sounds that have nothing to do with occupancy, such as slammed doors and street noise, and they require relatively high sound levels (higher than a typical quiet office) to activate. Few if any sensors use only acoustic technology, but some sensors combine PIR with acoustics to increase reliability.

Hybrid sensors. Hybrid, or dual-technology, sensors incorporate features of different sensor types in one unit. The most common combination of sensor types is that of PIR and US sensors, which take advantage of the PIR units' resistance to false triggering and the sensitivity of ultrasonics.

Advanced sensor features. Sensors are always improving. For example, some new infrared sensors are equipped with double-eye sets to minimize blind spots. Ultrasonic "boosters" enable a single wall-mounted sensor to cover very large or oddly shaped rooms without additional wiring. When connected to a heavy-duty relay, a single boosted US sensor can effectively cover several thousand square feet, needing only hand or arm motion to trigger it. Other switches combine occupancy sensors with dimmers to take advantage of the energy-saving potential of both technologies. Line-voltage sensors have recently become available that make it practical to install occupancy sensors on individual fixtures in applications such as high-bay facilities, and they are reliable, cost about half as much as earlier sensors, and install in half the time. Finally, new "smart" sensors use microprocessors to provide automatic sensitivity or time adjustment and incorporate sophisticated analysis of the detected signals to improve operational reliability. Unlike a traditional sensor that uses a preset time-delay, after which a sensor that hasn't detected any presence will shut off, a smart sensor can "learn" the changing activity levels and habits of building occupants and adapt the time delay to improve performance.

How to make the best choice
this section

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.
What's on the horizon?
this section

New wireless sensors are becoming more widely available that not only simplify retrofits but could also make the sensors more effective. For example, a wireless system makes it easier to replace malfunctioning sensors or adjust their positions. Wireless sensors cost more, but even so, wireless solutions can still be attractive in some applications. These include small buildings that have not yet taken advantage of any control technologies, office buildings that frequently reconfigure space for new tenants, and old buildings that are being converted to modern office spaces. Also, at least one company is developing a solar-powered wireless sensor.

Who are the manufacturers?
this section
  • The Watt Stopper The Watt Stopper manufactures a complete line of energy efficient and intelligent lighting, HVAC, and office power control products.
  • Sensor Switch, Inc. Sensor Switch manufactures passive infrared occupancy sensors, daylight control devices, and most recently, passive dual technology sensors.
  • Cooper Controls In 1977, Novitas, which is now part of Cooper Controls, invented and produced the first sensor for lighting control. Today, it provides more than 2,000 large commercial, institutional, and governmental organizations with controls for lighting and HVAC.
  • Leviton Leviton produces controls for commercial, industrial, and residential applications, including box-mounted, surface-mounted, and ceiling-mounted sensors that control a variety of light sources. Leviton occupancy sensors are available with passive infrared or ultrasound area monitoring. The line includes devices for indoor and outdoor applications and features products that switch lighting themselves or that command a separate switching relay.
Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of your service provider’s various technology-related programs.
E Source Business Energy Advisor

Your one-stop source for actionable advice on proven business energy management strategies. The E Source Business Energy Advisor (BEA) is a library of information that integrates with your existing website and provides sector-specific energy advice for a wide variety of business types and technologies. By providing this valuable information on your website, you can improve engagement with your business customers, help them make the case for energy efficiency, and drive them to your efficiency programs. BEA also serves as a valuable resource for account managers, business contact center representatives, and demand-side management program managers.