1. An evaluation method for determining a power reduction due to ageing of a photovoltaic module, comprising: measuring an electric variable representing a power performance of the photovoltaic module using a data logger in one or more predetermined time periods within a given year, each time period defining a class; constructing power curves of discrete power values, each power curve constructed from the measured electric variable at a plurality of times of a day within each class, and the power curves constructed for a plurality of years; determining energy output values based on the constructed power curves for at least one class for each of the plurality of years using a computation component; 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 using the computation component.