A method for performance evaluation of mixed, sensible thermal energy storage (TES) systems is presented based on exergy efficiency. The exergy efficiency of the storing period, normally the part of a complete storage cycle having the most significant time and temperature dependence, allows the direct evaluation of the performance of sensible and mixed TES systems. The exergy efficiency of the storage period is evaluated in terms of a dimensionless temperature and a cooling coefficient, both of which relate the thermal performance of the storage with the duration of the storage period and two absolute temperature ratios (involving the environmental temperature and the initial storage temperatures for the charging and storing periods). The method presented is applicable to TES systems that undergo constant-pressure processes in a complete storage cycle. An illustrative example is presented to highlight the benefits of the method and its exergy basis for TES evaluation and comparison. It is believed the method will assist those in industry and elsewhere to optimally design and construct TES systems.