A method for determining a power reduction due to ageing of a photovoltaic module includes measuring a variable associated with a performance of the module in one or more time periods within a given year, each time period defining a class, and constructing curves of discrete values, each power curve constructed from the measured electric variable at a plurality of times of a day within each class, and the curves constructed for a plurality of years. The method includes determining energy values based on the constructed curves for at least one class for each of the plurality of years, and determining a power reduction of the photovoltaic module with respect to one or several previous years by calculating a difference between the energy output values of a given class in a particular year and the energy output values of the given class in a year previous to the particular year.