The determination of optimal illumination conditions in buildings is of great interest both for reducing energy consumption and for exploiting solar resources with greater efficiency and sustainability. The most commonplace method of estimating daylight is the luminous efficacy approach, using the more widely measured solar irradiance. In this present study, a new model of diffuse luminous efficacy over a horizontal surface is proposed. A comparative study of twenty-two classic models is presented, to obtain diffuse illuminance, using both, the original mathematical models and the adapted models with local coefficients, in order to determine the most suitable models for Burgos, a city located in north-western Spain. With this purpose in mind, twelve models are selected for all sky conditions, five models for modelling clear sky, two for partly cloudy sky, and three for overcast sky. These twenty-two models are then compared with the new model both for all sky conditions and for particular sky conditions (clear, partly cloudy, and overcast). The behaviour of the new model showed greater accuracy than most of the classic models under analysis. Hence, the advantage of the diffuse luminous efficacy model that can be applied both to all sky and to particular sky conditions. (C) 2019 Elsevier Ltd. All rights reserved.