The recently published Olmo model estimates global irradiance on inclined planes. It only requires the horizontal global irradiance and the sun's azimuth and elevation as input parameters. As a consequence, it does not contain empirical coefficients that need determination for each location. Olmo and co-authors found a good agreement between predicted and experimental values obtained at their measuring site when a factor accounting for ground-reflected radiation was introduced in the model. In the present paper, the Olmo model is assessed in Madrid for vertical planes facing north, south, east and west. The model gives a root mean square error of congruent to27%, when all data are used. With the data for the north-facing plane it becomes worse, i.e. congruent to52%. In general, the model performs badly when the surface receives only diffuse radiation. This is the case when the vertical plane does not "see" the sun's disk, or when for intermediate or overcast skies no direct radiation is incident on both, vertical and horizontal surfaces. A distinctive feature, observed when the model is used with the data for the south, east or west-facing planes, is that many points in the plot of the calculated versus measured global irradiance clearly follow an undulating pattern.