[en] Highlights: ► Estimation of diffuse horizontal and vertical (N, S, E, W) illuminance at Madrid. ► Monthly mean of hourly values are obtained based on a data series of 26 years. ► Tables representing the climatic behavior of diffuse illuminance are given. ► Empirical luminous efficacy models with local coefficients are used. ► The influence of the nubosity on the illuminance levels have been analyzed. - Abstract: The overall objective of this work is to provide diffuse illuminance availability at Madrid (Spain) through a statistical analysis of illuminance values corresponding to a long-term data series. The illuminance values are obtained from irradiance measurements by means of different empirical models for luminous efficacy. The values of diffuse illuminance on a horizontal and on vertical surfaces facing the four cardinal points are estimated and the different aspects related to daylight availability in an area with specific climatic conditions are analyzed. The experimental data consist of global and diffuse irradiance measurements on a horizontal surface provided by the National Meteorological Agency in Spain (AEMET) for Madrid. These data consist of hourly values measured in the period of 1980–2005. The statistical results derived correspond to a daylight typical year for the five surfaces considered. This information will be useful to building experts to estimate natural illumination availability when daylighting techniques are applied in building design with the main aim of electric energy savings.