Predicting diffuse illuminance on vertical surfaces at Arcavacata di Rende (Italy) for various types of sky

The diffuse natural illuminance of variously oriented vertical surfaces was calculated by nine models under various sky conditions, subdividing the experimental data into eight types of sky, on the basis of the value of the Perez sky clearness index parameter. The calculated data were compared with the experimental data measured over a complete year at Arcavacata di Rende (Italy) (latitude 39°21′N, longitude 16°13′E).The comparison between calculated and experimental data showed that the best models appear to be an author's model, which assumes a constant value of the luminous efficacy of the radiation for each surface in each sky clearness index bin and a version of a Perez model whose coefficients were recalculated for each surface orientation. If all the data are considered together, without distinguishing between the different types of sky, almost all the models, for the various surfaces, present low errors and behave in a quite similar manner.