Luminous efficacies are determined at Madrid for North, South, East, West facing surfaces in two ways: by taking into account all the global illuminance and irradiance values available, and by considering data for each of three sky categories as defined from values of the sky clearness index epsilon ＇ and the sky brighteness index Delta. Both methods are compared, and for epsilon ＇ < 1.23 (overcast skies) the second method is found to be more accuarate than the first.