In this paper, the analysis of the local exergy costs of a cullet glass heated by microwaves in a cubical cavity activated by a susceptor is presented. The analysis is based on a previously validated 3-D electromagnetic model, but goes further by applying the concepts of local exergy efficiency and local unit exergy consumption, what enables a local analysis (in time and space) of the process efficiency. Furthermore, local exergy cost quantifies in detail the path of the exergy cost formation during microwave heating, which is determined by the local irreversibilities taking place in this transient process. Four different susceptor positions have been also compared, in order to find out not only which one is the most efficient but also to justify in detail this result by the time and space evolution of efficiency, unit exergy consumption (both external microwave power and conduction contributions) and unit exergy cost. The best conclusion of the paper is that the local exergy cost approach can contribute to the design of more efficient energy conversion systems, as it could be noted in its application to a complex process like this 3-D example of microwave cullet heating.