Point-source Perez illuminance model: Dependence of coefficients on surface orientation at Madrid

Abstract

The point-source model, the most simplified version of the original model developed by Perez to estimate irradiances and illuminances on tilted planes, has been assessed using values of mean hourly illuminance obtained during a period of 14 consecutive months at Madrid. When the model is used to fit data for all the vertical surfaces facing north, east, south and west, it performs better with local coefficients than with those given by Perez. The model's accuracy is considerably increased by the use of local coefficients derived independently for each orientation. In this case if coefficients are obtained separately for the data obtained when the surface 'sees' or does not 'see' the sun, a further, though smaller, increase in accuracy results.