Irradiation data, recorded on vertical surfaces facing north, south, east and west and on a horizontal surface every ten minutes during daylight hours from January-December 1992 in Valencia, Spain, have been compared with estimated solar irradiation from inclined-surface models. Results show that Hay＇s model most accurately reproduces the variation in irradiation on all vertical surfaces. Hay＇s model has been used to find the hourly variation in the optimum tilt angle for a South-facing solar collector in Valencia, Spain, and also to calculate the yearly average of this angle. This method has been compared with the results provided by another model that uses average monthly values of daily irradiation derived from the same experimental data, to calculate average monthly values of the optimum tilt angle. The results show that the method involving monthly averages is more accurate and easier to work with.