The IEC 61853 standard series define the Climate Specific Energy Rating (CSER) which can be used within the Photovoltaic (PV) community as a reliable and accurate tool to predict a PV module’s energy output for specific climate and allows to differentiate between PV modules according to their specific performance for representative locations. The CSER power rating parameter is assumed to represent the PV device performance in real-world operation. So far, research has mainly focused on the implementation of different algorithms to determine the input parameters and to calculate the CSER values based on indoor measurement performance rating data. There is lack of information and technical specification how to obtain these parameters from outdoor field operating PV module data that reflects better the module performance behaviour. In the present study, we analysed long-term outdoor field data for a period of 36 consecutive months of a PV module in order to understand how the length (and seasonality) of the data acquisition period impacts the reliability of irradiance and temperature power matrix values, particularly in relation to CSER calculations. The PV outdoor performance data has been validated against its indoor counterpart. The results concluded that a period of 9 or 12 months provides a good compromise between accuracy and time-resources. This may be reduced to 6-month provided the meteorological conditions are sufficiently diverse in this time frame. The study provides important insights to the PV community since the CSER parameter may become a compulsory requirement for each PV module produced, imported or sold in Europe if the potential EU Ecodesign Directive and the Energy Labelling Regulation will be implemented.