Estimation of the rates of power degradation of PV modules (%/year) is important for assessing the long-term reliability of PV systems. The degradation rate also affects the lifetime of the module, and hence the LCOE. The ‘All-India Surveys’ conducted by NCPRE at field sites across India indicate that modules in the ‘Hot’ climates of India degrade faster than those in the ‘Non-Hot’ climates. Whenever the exact initial power of PV modules under STC (Standard Test Conditions) is not known, various methods based on the nameplate power are used for calculating the degradation rate. NCPRE has performed analysis using a controlled study of artificial datasets and insights are provided for appropriate interpretation. It is observed that the distribution of calculated degradation rates is an artifact caused by the distribution of initial ‘True’ powers while using the nameplate-based methodology. Even when all modules from a site are assumed to have exactly the same ‘True’ degradation rate, one still sees a distribution in the calculated degradation rates using the modified nameplate based methodology. The origin of this distribution in the calculated degradation rates is the initial distribution in the actual powers of the modules. The spread in the calculated degradation rates reduces with time, and this reduction is faster in the initial years of field exposure and the rate of spread reduction reduces over time. This can be explained by the fact that the error between the actual and nominal power gets divided by the age of the PV system, hence its contribution to the linear degradation rate distribution reduces with age. If the spread is found to remain the same or increase in subsequent years, it is likely to be due to the fact that a subset of modules has started to degrade at a different rate than the rest of the modules. This often shows up as a distinct emerging peak in the distribution of calculated degradation rates - indicating the emergence of a different degradation mechanism. A similar widening of spread in a few sites of the All-India Surveys (AIS) was observed. The distributions observed in one of the sites of the AIS is shown below for the data from the 2016 and 2018 surveys. On further investigation of this site, it was found that encapsulation browning was observed in some of the modules. This was a new degradation mechanism which emerged after some years. This resulted in a new peak, which is clearly seen in the 2018 histogram, which is not present in the 2016 data, and this new degradation mode increases the mean and median of the 2018 data set. This paper by R. Korgaonkar, A. Kottantharayil, J. Vasi and N. Shiradkar was presented by Ph.D. student Ruchita Korgaonkar at the Asia PVSEC conference in Numazu, Japan on November 14, 2024.