This work outlines the evolution of measured module parameters (power P-max, short circuit current I-sc, open circuit voltage V-oc) vs labelled module parameters of crystalline silicon PV modules over a number of decades at the European Solar Test Installation (ESTI) at Ispra, Italy. Manufacturers' tolerance in nameplate power has changed over the years with many claiming +/- 10% in the 80s and 90s. The declared tolerance over recent years has been reduced to +/- 5% then +/- 3% and today values of -0/+3% are quite common. To know the declared tolerance of modules in an operational system is useful additional information when investigating the aging of PV systems, when the original module power may either be unknown or missing with the passage of decades. Therefore, researchers have often to rely on the labelled power for the starting point of the system. Measurements at the ESTI laboratory have been performed since the early 1980s. A comparison of those modules which incorporated either a nameplate (or an associated datasheet, flash report or online information) to their measured output parameters (P-max, I-sc and V-oc) has been carried out. The impact of these differences from an economic point of view has been also analysed. This work presents some selected results from a set of 992 crystalline silicon based modules, extracted from various time periods from 1982 to 2014.