Volume 13 Issue 1
July 2015    
ballast - A device required by electric-discharge light sources such as fluorescent or HID lamps to regulate voltage and current supplied to the lamp during start and throughout operation. compatible ballasts - An abbreviated list of common ballasts that will provide the necessary circuitry for a photosensor to operate correctly. Other ballasts may also be compatible; contact the photosensor manufacturer for details. continuous dimming - Control of a light source's intensity to practically any value within a given operating range. capacitor - A device used in electric circuitry to temporarily store electrical charge in the form of an electrostatic field. In lighting, a capacitor is used to smooth out alternating current from the power supply. time delay range - For motion sensors, the range of time that may be set for the interval between the last detected motion and the turning off of the lamps. lamp - A radiant light source. luminaire - A complete lighting unit consisting of a lamp or lamps and the parts designed to distribute the light, to position and protect the lamp(s), and to connect the lamp(s) to the power supply. (Also referred to as fixture.) frequency - The number of cycles completed by a periodic wave in a given unit of time. Frequency is commonly reported in cycles per second, or hertz (Hz). electromagnetic interference (EMI) - The interference of unwanted electromagnetic signals with desirable signals. Electromagnetic interference may be transmitted in two ways: radiated through space or conducted by wiring. The Federal Communications Commission (FCC) sets electromagnetic interference limits on radio frequency (RF) lighting devices in FCC Part 18. electronic ballast - A ballast that uses electronic components instead of a magnetic core and coil to operate fluorescent lamps. Electronic ballasts operate lamps at 20 to 60 kHz, which results in reduced flicker and noise and increased efficacy compared with ballasts that operate lamps at 60 Hz. illuminance - The amount of light (luminous flux) incident on a surface area. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux, although for convenience 10 lux commonly is used as the equivalent. dimming ballast - A device that provides the ability to adjust light levels by reducing the lamp current. Most dimming ballasts are electronic. power - The power used by a device to produce useful work (also called input power or active power). In lighting, it is the system input power for a lamp and ballast or driver combination. Power is typically reported in the SI units of watts. photosensor - A device used to integrate an electric lighting system with a daylighting system so lights operate only when daylighting is insufficient. lux (lx) - A measure of illuminance in lumens per square meter. One lux equals 0.093 footcandle. nadir - In the lighting discipline, nadir is the angle pointing directly downward from the luminaire, or 0. Nadir is opposite the zenith. driver - For light emitting diodes, a device that regulates the voltage and current powering the source. photovoltaic (PV) - Photovoltaic (PV) cells produce electric current from light energy (photons). PV cells are joined to make PV panels. hysteresis - The dependence of the output of a system not only on its current input, but also on its history of past inputs. The electric light level set by a photosensor with hysteresis, for a certain photocell input signal, depends on whether that photocell signal is increasing or decreasing. Hysteresis provides stable operation in switching photosensors but is undesirable in dimming photosensors.

Do wireless occupancy sensors have sufficient energy to operate?

NLPIP evaluated the battery life of three wireless occupancy sensors and the PV energy harvesting characteristics of the two of these sensors equipped with PV modules.

As shown in Table 4, the Lutron occupancy sensor was not equipped with PV and was powered solely by a replaceable battery. Both the Leviton and WattStopper occupancy sensors employed PV with an electric double layer super capacitor for energy storage, and both of these devices had an option of using a non-rechargeable replaceable battery as well.

Table 4. Wireless occupancy sensor battery specifications and presence of PV modules.

PV modules
(3V lithium)
Type/Size Required or
Yes ½ AA Optional 950 mAHr
No CR123A Required 1500 mAHr
Yes CR2032 Optional 240 mAHr

NLPIP estimated the battery life of each occupancy sensor by measuring the steady-state power and RF transmission energy and then using the equations and assumptions shown in Appendix: Detailed Methodology.

The results show that the Lutron sensor battery is projected to last for 16 years, the Leviton battery for 22 years, and the WattStopper battery for 8 years. The estimated battery life of the WattStopper device is shorter than the other two devices because of its smaller, coin cell battery.

NLPIP also characterized the energy generation by the PV modules incorporated into the Leviton and WattStopper occupancy sensors.  The PV modules were illuminated with a phosphor-converted white LED module and the current flowing into the super capacitor was measured. NLPIP found that the PV cells on both the Leviton and WattStopper products produce approximately 100 nA of current per lux.

Based on this PV current generation and the sensors’ average power demand measured by NLPIP, both sensors require 1200 to 1700 lux-hours per 24-hour period (e.g. 100 to 140 lux for 12 hours) to maintain operation without a battery. However, assuming an average work plane illuminance of 300 lux, a 5:1 task:ceiling illuminance ratio (NLPIP 2007) and 12 hours per day of lighting, the sensor would receive only 720 lux-hours per 24 hours, which would not be sufficient to maintain operation. Therefore, some commercial ceiling locations receive too little illuminance to keep the tested Leviton and WattStopper occupancy sensors operational without a battery.

NLPIP’s calculated illuminance requirement is higher than Leviton’s specification for its wireless occupancy sensor of 645 lux-hours per 24 hours (specified as 20 foot candles for 3 hours every 24 hours) and WattStopper’s specification of 860 lux-hours (specified as 20 foot candles for 4 hours to operate 24 hours). A possible reason for the difference between NLPIP’s calculations and the manufacturers’ specifications is that NLPIP measured PV electricity generation with a white LED, whereas the manufacturers may have measured with daylight, which provides higher electricity production per lumen. If this is the case, NLPIP suggests that the white LED is more representative of an office environment.

NLPIP also estimated the duration of the capacitor charge for the two tested sensors when PV is first used to fully charge the internal capacitor and then the sensor is left in the dark without a battery installed. The same battery-depletion calculation methodology mentioned earlier was used for this analysis, except motion detection was excluded. The capacitor used in the Leviton device is estimated to power the sensor for two days in the dark without a battery.  NLPIP’s estimate is consistent with Leviton’s specification that the sensor has an operating life of 48 hours when starting at full charge. This imposes a limitation on using this device in places that might not be illuminated by daylight or electric lighting for several days in a row, such as any interior space left unlit over a long weekend. NLPIP estimates that the WattStopper sensor would operate for eight days in the dark, which is longer than WattStopper’s specification of 72 hours. If the sensor is located in a space not illuminated by daylight or electric lighting for a longer period of time (extended darkness), then the sensor could become inoperable.

As discussed above, NLPIP identified two concerns about wireless sensors that rely on only PV for their energy: the potential to receive inadequate illuminance for PV charging and the possibility of energy depletion due to extended darkness when relying on PV. Therefore, NLPIP recommends installing batteries in wireless sensors and properly disposing of the batteries when they are depleted. Building maintenance staff should replace all of the wireless sensor batteries on a defined schedule.


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