The development of high-efficiency energy systems is a pressing issue nowadays, motivated by economic, environmental, and social aspects. Trigeneration systems allow for the rational use of energy by means of appropriate energy integration and provide greater operational flexibility, which is particularly interesting for buildings, often characterized by variable electricity, heating, and cooling demands. The benefits of trigeneration systems can be enhanced by the incorporation of thermal energy storage (TES), which decouples production and consumption. This paper analyses the operation of a simple trigeneration system including TES. The optimal operation is obtained by a linear programming model that minimizes the total variable cost. A thermoeconomic analysis based on marginal cost assessment of the internal flows and final products of the system is carried out, allowing to explain the optimal operation of the system and the role of the TES in achieving the optimal solution. The analysis unravels the marginal cost formation process, presenting a clear route from the final products obtained to the resources consumed. This information can aid the design of new plants, the retrofit of existing ones, and the operational management to achieve the minimum operational cost. (C) 2017 Elsevier Ltd. All rights reserved.