This paper presents a universal model of luminous efficacy for direct solar radiation on a horizontal surface. The model is applicable to all sky conditions and is based on data obtained from satellites and available via web servers. Solar radiation data from 10 locations in Europe and North Africa has been used to obtain four functions for luminous efficacy (K) against the sole independent variable solar altitude (alpha). Additionally cloud amount (C) was been used to obtain four other functions. All were used to accurately estimate illuminance for the 10 originating locations; for four locations based on satellite data; and for a further four based on measured data. A statistical assessment showed that three models performed best, namely, K against 1/alpha, K against sin a, and K against C/alpha. Comparison between results from the proposed models, and those produced using three previously published models, indicate that the former produce more accurate estimates of luminous efficacy. The satellite based approach makes daylight data available in locations remote from current measurement sites. (C) 2010 Elsevier Ltd. All rights reserved.