As photovoltaic (PV) penetration of the power grid increases, accurate predictions of return on investment require accurate analysis of decreased operational power output over time. The degradation rate in PV module performance must be known in order to predict power delivery. This article presents the degradation rates over 10 years for seven different PV systems located in England, Scotland, and Ireland. The lowest PV degradation rates of −0.4% to −0.6%/year were obtained at the Irish PV sites. Higher PV degradation rates of −0.7% to −0.9%/year were found in England, whereas the highest degradation rate of −1.0%/year was observed in relatively cold areas including Aberdeen and Glasgow, located in Scotland. The main reason that the PV systems affected by cold climate conditions had the highest degradation rates was the frequent hoarfrost and heavy snow affecting these PV systems, which considerably affected the reliability and durability of the PV modules and their performance. Additionally, in this article, we analyse the monthly mean performance ratio (PR) for all examined PV systems. It was found that PV systems located in Ireland and England were more reliable compared to those located in Scotland.