A new method was developed to estimate the cloud optical depth (τ_c) from photovoltaic (PV) power measurements under overcast sky conditions. It is the first fully physical and universally applicable method utilizing directly PV power measurements. It exploits the recent advances and real-time availability at global scale of aerosol properties, downwelling shortwave irradiance and its direct and diffuse components received at ground level under clear-sky conditions, ground albedo and extraterrestrial irradiance, altogether provided by the Copernicus Atmosphere Monitoring Service (CAMS) radiation service. In addition to CAMS data, wind speed and air temperature from European Centre for Medium-Range Weather Forecasts (ECMWF) twentieth century reanalysis ERA5 products are also used as inputs. An algorithm for selecting overcast sky conditions has been designed too. The estimates have been compared to different data sources of retrievals at four experimental PV sites located in various climates. When compared to retrieved from ground–based pyranometer measurements serving as reference, the correlation coefficient is greater than 0.97. The bias ranges between –3 and 4, i.e., −8 % and 12 % in relative value. The root mean square error (RMSE) lies in the interval [3, 8] ([9, 21] % in relative value). When compared to satellite–based retrievals from Meteosat Second Generation (MSG) and Moderate Resolution Imaging Spectroradiometer (MODIS), both relative errors become comprehensively greater. Nevertheless, our method remarkably reduces the relative bias and RMSE, by up to 10 % and 20 % respectively, compared to the existing state-of-the-art approach. This work demonstrates the accuracy of the method and clearly shows its great potential use whenever PV power measurements are available.