PV system designers need an estimation of the temperature at which a system will be operating in the
field, in order to evaluate the losses due to thermal effects. To that purpose the IEC Standards for qualification of
modules point out at the NOCT, the Nominal Operating Cell Temperature, as an useful parameter. Anyway NOCT
measurements require an accurate elaboration of data, since they are to be referred to quasi stationary conditions, and
sometimes the correct interpretation of some clauses of the Standards are not so easy. Because NOCT measurements
put severe restrictions on the specific mounting and measurement conditions some laboratories as the ESTER
laboratory of the University of Rome Tor Vergata measure instead the NOST, the Nominal Operating Specific
Temperature. So a better and a deeper understanding of what can be effectively used as the operating temperature is
highly required. The paper intends to present and compare results of NOCT and NOST measurements, obtained
respectively by two different laboratories located one, the ENEA laboratory, in the Southern Italy, near Naples, and
the other in Rome, in the Central part of Italy.