There are a number of models available to estimate global irradiance or illuminance on inclined
surfaces from irradiance or illuminance on a horizontal surface, such as Refs. [1–4]. These models
require information of the global and the direct or the diffuse irradiance or illuminance on a
horizontal surface. Usually, the models are tested with data obtained for vertical surfaces, because
the departure from horizontal surface values is the largest in this case. An example is the anisotropic
model by Perez [3], regarded as one of the most reliable to estimate both irradiance and
illuminance on inclined surfaces, as confirmed, among others, by Robledo and Soler [5–8], Utrillas
and Martinez Lozano [9] and Li and Lam [10] for vertical surfaces. However, if this model is used,
a large number of site and orientation dependent coefficients have to be determined, even for the
simplest version [5–10].