This study investigates the performance of thermoelectric devices, specifically thermoelectric legs (TML), by using COMSOL Multiphysics simulations. Copper thermocouples having copper composite materials are investigated. The research uses finite element analysis to model heat and electric energy transfer within the thermoelectric system by considering whole surface temperature, electric potential difference, and iso-surface temperature. Simulation results indicate that the performance of TMLs in both mm and nm scales can be evaluated through the electric conductivity, thermal conductivity, and Seebeck values. The study explores impacts of composite material properties on thermoelectric device efficiency and highlights key challenges related to copper thermocouple integration and the complex behaviour of copper composite materials. This approach provides valuable insights for design of more efficient thermoelectric devices and addresses common issues encountered in experimental performance analysis.