Short- and long-term outdoor performances of the photovoltaic (PV) modules are extremely important in reaching their economic viabilities. Such information is very valuable for the investors, customers and researchers of PV technologies. Two different procedures are possible, and both are used by many researchers. However, to the best of our knowledge, there is no comparative investigation to evaluate these two procedures. One of the methods uses instantaneous I-V curves of PV modules taken in a specified instant of time, while the others simply use the integrated energy values of the input and the yield of PV modules. In fact, both have advantages and disadvantages. In the present work, we outlined the advantages and disadvantages of both methods and compared them using the data of two different cell-type modules which are installed at GuNAM's outdoor test facility. In the analysis, the data that we used are taken under the same outdoor test conditions and ambient weather conditions. The outcomes of these preliminary analyses can be summarized as follows: (1) The procedure that uses instantaneous I-V curves seems dependent on the specified time intervals and ambient weather conditions at that instant; (2) accordingly, we will propose how to determine an optimal time interval to reach better performance results; (3) using the integrated values, we achieved the variations in performances with different integration time periods; (4) preliminary comparisons of these two procedures resulted in key conclusions on performance and degradation calculations of PV modules.