In the present work, we have undertaken a study of the luminous efficacy of diffuse solar radiation incident on vertical surfaces facing north, cast, south and west for all sky conditions and mean hourly values of diffuse irradiance and diffuse illuminance. We have developed luminous efficacy models, of the type previously obtained for a horizontal surface, assuming that the surface slope will qualitatively influence illuminance and irradiance in the same way. Taking into account that the models obtained show that there is little variation of the luminous efficacy with the independent variables, an approximation to these models has been proposed that considers a constant value for the luminous efficacy of diffuse solar radiation. Statistical assessment shows that this approximation offers reasonable results for north, east and west vertical surfaces. When statistical assessment of the models is performed, it is observed that the values of the statistical indicators largely depend on the possibility for the surface to "see" the solar disk, as determined from geometrical considerations. For both types of models, better values of the statistical indicators are obtained when the surface sees the solar disk than when it does not.