Lighting energy savings from daylight: Estimation at the sketch design stage

Abstract

To assess the energy implications of window design requires inter alia a knowledge of the effects of interior daylight on electric lighting use. Current methods for predicting lighting energy savings are unsuitable for use at the initial design stage because they require details of lighting layout, control sensor locations and window shape and position. Average daylight factor has been recommended for use as a daylighting criterion at the sketch design stage because it is easy to calculate and can be related directly to glazing area. This paper shows how lighting energy savings under both photoelectric (switching or dimming) and manual control can be predicted from the average daylight factor in the space. The new methods dispense with the need to calculate point daylight factors, and do not require a knowledge of luminaire layout or control sensor positions. Although not directly addressed in this paper, the link with glazing area enables the thermal and lighting energy consequences of windows to be evaluated and compared at the early stages of design.