Daylight and solar data

In building design the ability to predict the effects of daylight is of increasing importance. Daylight can be an important factor in building energy efficiency; in some buildings lighting may account for half the energy cost. This paper describes the weather data that are available for daylight prediction.  First of all the requirements for data are evaluated. For many energy applications, the key quantity is the percentage of the working year a given design illuminance is exceeded by daylight. This requires external daylight illuminance data, together with a knowledge of sky luminance distribution. The paper then outlines the available data on daylight in the UK, including the Meteorological Office measurements and more recent data from Garston and Nottingham. These do not cover the whole of the UK, and so the paper also describes prediction methods to evaluate daylight availability where no measured data exist. These may be simple formulae based on solar altitude; complex models which can also predict sky luminance distribution; or luminous efficacy values to obtain daylight data directly from solar radiation data.