Investigating the thermal impact including the fluctuations of the solar irradiance and ambient temperature of photovoltaic (PV) systems is a topic of great interest by industry and policymakers, due to the limited case studies reported so far by the PV research community. Therefore, this article presents the analysis of 8000 PV systems distributed across England using the well-known metric, monthly performance ratio (PR). The PV systems were operated over five years, while the PR is calculated using the newly developed model by the US national renewable energy laboratory (NREL). Remarkably, it was found that the average monthly PR for all examined PV systems is equal to 85.74%, where the Midlands region in the UK has the highest monthly PR of 88.12%. We have also investigated the seasonal thermal impact on the performance of PV systems, where it was concluded that Spring and Summer seasons intend to have higher monthly PR compared to Autumn and Winter. Finally, a detailed experiment of three different PV modules affected by various hotspots, including cell-based and string-based, will be comprehensively discussed.