The object of this study is the durability / degradation evaluation of a 26-year-old photovoltaic (PV) monocrystalline silicon (c-Si) system located in Phoenix, Arizona (a hot-dry desert condition). This first of a kind neighborhood solar power plant, called Solar One, was built by developer John F. Long. This 4000-frameless-module bipolar system was originally installed with a rated capacity of 175 kW (17° fixed south tilt). This paper presents only the degradation rates of the modules, and the degradation and failure modes of this power plant are presented in another paper of this conference. The system was shown to degrade approximately at a rate of 2.3% per year with no apparent potential induced degradation (PID) effect in either polarity. The power plant is currently operating at less than 40% (<70 kW) of its rated capacity (175 kW). The resulting study showed that degradation was a direct result of several factors including: Module construction materials, module design, vandalism, installation methods and hot-dry environmental conditions of Phoenix. Ultimately, intensive encapsulation browning (leading to current loss - largest cause for power loss) and higher series resistance (potentially due to solder bond fatigue leading to fill factor loss) were determined to be the primary causes for the degradation loss.