Simple expressions for predicting energy savings from photo-electric control of lighting

Abstract

The cost-effectiveness of different types of photo- electrically controlled lighting systems is being examined at the Building Research Establishment. Using published data on availability of daylight, equations are derived to predict the maximum possible savings in artificial lighting use that can be achieved using photo- electrically controlled dimming or on/off systems. The equations are applied to two deep-plan spaces and figures are given for the relative maximum savings in artificial lighting use of three control systems: on/off, dimming and mixed. The savings range from 20%, to over 40%, of the energy used for uncontrolled lighting, assuming that a horizontal illuminance of 500 lux is maintained throughout the spaces. The effects of real variations in artificial lighting use are discussed.