The transition to a carbon-reduced future for one of the most energy-intensive actors, the building sector, requires the development of appropriate tools and methods. One such approach is local energy communities (LECs), especially thermal LECs (TLECs), which provide a promising vector towards that transition. LECs exploit energy users as key actors in the energy production process. However, their formation, creation, and continuation are still an ongoing endeavor. Many research efforts focus on creating and continuing LECs in an economic, legal, and incentivized manner, sparsely addressing the formation process. In this Part A, a collective tool for decision-making for potential TLECs is presented. The current study proposes a unified approach to classify the prosumers of energy (consumers who both produce and consume energy) using conventional methodologies (RenewIslands, Kaya Identity). A case study is presented in a fully operating LEC in Kimmeria, Greece, in which both the traditional methods of classifying users are applied as well as the proposed methodology, in comparison. The results indicate a significant improvement to the conventional solutions, which tend to overestimate the needed equipment, leading to extensive installation and operational costs.