1. An evaluation method for determining a power reduction due to ageing of at least one photovoltaic module, comprising: measuring an electric variable that changes over a period of time using a data logger, the electric variable comprising one or more of cell current, cell voltage and/or cell power associated with the at least one photovoltaic module, and storing the measured electric variable in a data bank; fixing at least one class that is compared year by year and within which a daily power curve or portions of the daily power curve of the photovoltaic module derived from the measured electric variable are compared to each other, the class corresponding to a defined time period within a given year, determining energy output values that are compared year by year for the at least one class from a comparable power curve associated with the respective at least one class through the electric variable delivered by the photovoltaic module so that energy differences are determined based on the classes, using a computation component, indicating a power reduction of the photovoltaic module with respect to one or several previous years from the energy output value by calculating a difference with respect to the energy output values of the years to be compared using the computation component, data of comparable days in the respective at least one class having a comparable power curve of the photovoltaic module being observed over several years.