Information on solar ultraviolet radiation (UVR) on the Earth’s surface is essential for fields such as health/materials sciences, and energy. UVR measurements are commonly taken on a horizontal plane, which is also the reference plane for the available database estimates. However, for many applications, such as water treatment, information on UVR in the tilted plane may provide more insightful results. There is a lack of studies in the literature that address the problem of UVR on inclined surfaces. In this study, an isotropic transposition model was used to estimate UVR in tilted surfaces in six different cities along the Chilean territory with potential applications in solar water treatment. In this context, isotropic refers to the diffuse and direct radiation component modeling. In addition, mathematical models were developed to forecast Chilean cities’ monthly and yearly optimum tilt angles. The efficiency of the solar photo-Fenton process for treating paracetamol-contaminated wastewater using a compound parabolic collector photoreactor tilted at different angles, was evaluated through simulation. The gains increased at the highest latitude were 30.13 % for monthly, 21.05 % for seasonal, and 9.23 % for yearly adjustments. The empirical models developed were found to be highly accurate (R2 ≥ 0.81, RMSE ≤ 0.98°, MAPE ≤ 2.70,% SSRE ≤ 0.01°, RSE ≤ 0.02°, and MBE ≤ 0.001°). Using the local latitude as the photoreactor tilt angle (the current general design strategy) resulted in lower efficiency (m3 of wastewater treated per month) than using the optimal tilt angle for the month with the lowest UVR (winter), and the entire year. These results highlight the importance of fine-tuning the photoreactor tilt angle locally and, consequently, the need to develop UVR models that account for this variable.