Although recent laboratory tests are showing promising progresses in the materials and production technologies of photovoltaic (PV) devices, the commercial PV modules do not show analogous impressive improvements. Therefore, a diagnostic approach, able to check the current state of health of already installed PV systems, as well as their trend of ageing, assumes a strategic importance. In this scenario, we introduce a thermography-based diagnostics able to provide a detailed, clear, and unambiguous information, thanks to a computer-aided investigation that is much deeper than the today available infrared analysis. The proposed approach allows a numerical and qualitative evaluation of each cell of the PV device. This Part I-Framework introduces the methodology, based on two main analyses. The first one (cell analysis) studies each single cell, while the second one (cluster analysis) focuses the attention on groups of PV cells. The framework is also characterized by a preprocessing in which the region of interest is extracted from the infrared image in order to focus the successive processing and analyses only on this area. The Part II-Platform and Results shows the cloud platform implementing the workflow (it automatically generates a comprehensive and detailed report), and discusses also several significant cases of study.