The performance of five different diffuse hourly irradiation models on tilted surfaces is examined using an assembled data set of measured hourly global and diffuse solar irradiation on horizontal and tilted surfaces in the Castile and Leon region and from surrounding stations. Estimated and measured diffuse solar irradiation values on a tilted surface have been compared using statistical tests based on root mean square error and mean bias error. From the results, it has been found that the Perez' 25degrees model presents the best overall calculated performance. The global solar irradiation on a tilted surface has been calculated using Perez' 25degrees model, and the results have been compared and plotted against the tilt angle for all year. The yearly mean solar irradiation isoline maps over tilted surfaces and over vertical surfaces with different orientations have been plotted, and the optimum tilt angle has been shown. The results show that the highest irradiation values of 5.0 kWh m(-2) are obtained in the centre of the region (Valladolid) with 30degrees or 45degrees tilt. There is a strong gradient from the centre to the north region.