The primary goal of this thesis work is to determine the activation energy for encapsulant browning reaction of photovoltaic (PV) modules using outdoor field degradation data and indoor accelerated degradation data. For the outdoor field data, six PV modules fielded in Arizona (hot climate) over 21 years and four PV modules fielded in New York (cold climate) over 18 years have been analyzed. All the ten modules were manufactured by the same manufacturer with glass/EVA/cell/EVA/back sheet construction. The activation energy for the encapsulant browning is calculated using the degradation rates of short-circuit current (Isc, the response parameter), weather data (temperature, humidity, and UV, the stress parameters) and different empirical rate models such as Arrhenius, Peck, Klinger and modified Peck models. For the indoor accelerated data, three sets of mini-modules with the same construction/manufacturer as that of the outdoor fielded modules were subjected indoor accelerated weathering stress and the test data were analyzed. The indoor accelerated test was carried out in a weathering chamber at the chamber temperature of 20°C, chamber relative humidity of 65%, and irradiance of 1 W/m2 at 340nm using a xenon arc lamp. Typically, to obtain activation energy, the test samples are stressed at two (or more) temperatures in two (or more) chambers. However, in this work, it has been attempted to do the acceleration testing of eight mini-modules at multiple temperatures using a single chamber. Multiple temperatures in a single chamber were obtained using thermal insulators on the back of the mini-modules. Depending on the thickness of the thermal insulators with constant solar gain from the xenon lamp, different temperatures on the test samples were achieved using a single weathering chamber. The Isc loss and temperature of the mini-modules were continuously monitored using a data logger. Also, the mini-modules were taken out every two weeks and various characterization tests such as IV, QE, UV fluorescence and reflectance were carried out. Activation energy from the indoor accelerated tests was calculated using the short circuit current degradation rate and operating temperatures of the mini-modules. The activation energy for the encapsulant browning obtained from the outdoor field data and the indoor accelerated data are compared and analyzed in this work.