Abstract A numerical study of energy conversion during microwave heating of cullet glass is presented by means of a combined electric and thermal model. Ceramic materials, such as glass, poorly absorb microwave radiation at temperatures below 300 °C. Thus, in order to obtain the simulation of microwave heating from room temperatures, the effects of a SiC susceptor were studied. Four different positions of the susceptor were simulated to improve that heating process. The numerical simulation of cullet glass and susceptor behavior under an electromagnetic field was analyzed by applying transient Maxwell’s equations, which were solved by the finite difference time domain (FDTD) method. Once the electromagnetic field in the waveguide and microwave cavity was computed, temperature inside of the applicator was determined by solving the heat transfer equations including microwave absorption, conduction and internal radiation terms. Parametric simulations showed that the susceptor position changed the electromagnetic and heat transfer patterns, and then the temperature field inside of the cullet glass. Therefore, energy consumption of heated cullet glass was also affected.