Photocontrols or “light level controls,” which reduce electric lighting in response to sufficient interior daylight illuminance, are the next frontier for saving energy with lighting controls. However, many recent anecdotal reports of photocontrol performance have been negative. Our hypothesis is that many of these negative reports are for photocontrol systems that have been applied to spaces that are sidelit with windows and that the success rate for photocontrols under skylights would be much higher. The rationale for this hypothesis is that skylighting is “easier” than sidelighting – the distribution of light from skylights remains fairly constant regardless of sky conditions or sun position. This paper describes site surveys of 32 photocontrol systems under skylights, the calculation methods used, and the findings of this study. The primary metric of photocontrol performance developed for this study is the “realized savings ratio,” or the ratio of the actual lighting energy savings over a two week period as measured by power monitoring to that predicted by a daylighting simulation using the SkyCalc software and weather station illuminance data over the same period. The realized saving ratio varied from 25% (little energy savings) to 156% with the mean realized savings ratio of 98%. In all but one case the realized savings ratio was greater than 60%. In many cases, realized savings ratios greater than 100% were self-reported as being due to manual switching. The results of this field study present compelling evidence that photocontrols under skylights reliably save energy. Introduction Energy efficient lighting design can be summarized in the following concepts: 1. Provide the appropriate light levels where they are needed. This is the foundation of task/ambient lighting design and is the basis of luminaire selection and placement. 2. Use the highest efficacy (lumens per Watt) source as is appropriate for the task. 3. Provide flexibility to adjust light levels. The need for light varies between people and varies over time or by task performed. Multi-level switching, multiple lighting circuits or dimming provide this flexibility. 4. Turn off electric lighting when it is not needed. Time clocks and timers automatically turn off lighting based on a schedule or a duration of needed light. 1 Heschong Mahone Group, Fair Oaks, CA 2 Southern California Edison, Irwinedale, CA 2004 IESNA Annual Conference Effectiveness of Photocontrols with Skylighting Occupancy sensors directly sense occupancy and turn off lights when areas are unoccupied. 5. Turn off or dim electric lighting in response to increasing levels of daylight in the space. Lights can be automatically controlled in response to daylight through the use of photocontrols or “light level” controls. This last element of efficient lighting design, photocontrols is the subject of this paper. The subject of photocontrols in interior spaces is quite timely in that they are rarely used, are not very well understood but have the capability of being the next big energy savings opportunity in lighting design. In well daylit spaces well-designed photocontrol systems can reduce lighting power by 50% 70% for at least 2,000 hours per year. Where are these daylit spaces? The California Title 24 energy standard describes the “daylit area” as being within 15 feet of perimeter windows or within a cone of light that expands the “footprint of the skylight” by 70% of the ceiling height in all directions. See Figure 1 for a graphical description of the daylit area.