Molten thermoelectrics are proven candidates to harvest waste-heat from high-temperature industrial operations in the form of electrical power. However to date, the role of thermal conductivity remains to be evaluated in order to evaluate the overall efficiency. Herein, possible modes of heat transfer and their analysis for such molten semi-conductor are presented. Experimental estimates of thermal conductivity obtained on a power generation thermoelectric device operated with molten tin sulfide at temperature in excess of 900 °C are then presented. Results of the simulation of the temperature profile, the fluid mechanic model and electrical power performance show a key role of natural convection and a minor role of radiative heat transfer, and indicate a possible variation in transport properties of tin sulfides around 1000 °C.