Theflexibility provided by conventional generating units is very important sincethese units are the main resource for satisfying the net load. Theflexibility potential ofthe generating units depends by their present operational state and their technicalcharacteristics such as operational range, ramping rates, start-up time, shut-down time,and the minimum up and down times. There are several indices in the literature toassess theflexibility of the system. In [5–7], aflexibility index is defined in order toquantify theflexibility of each type of generating unit. This index is mainly based onthe technical characteristics of each unit. Also, the calculatedflexibility index in [6]isincorporated in a unit commitment (UC) model to investigate the impact of generatorsflexibility in the day-ahead operational decisions. In [7], theflexibility indices of thegenerators are integrated in a dedicated unit construction and commitment (UCC) al-gorithm in order to determine the optimal investments inflexible generating units.Results show that the increase of the systemflexibility can handle more uncertainties inload and generation. At the same time though the greaterflexibility provided byconventional generating units results in an additional operating cost.ESSs are an emerging technology that can reduce the operational cost of the systemin valleyfilling and peak shaving applications, due to their ability to absorb powerduring low-cost hours and to inject power during high-cost hours [8,9]. Also, ESSs areveryflexible resources that can be used in compensating the intermittent and uncon-trollable character of RES. In [10], an ESS is incorporated in a UC model in order toevaluate the storage profits under a high penetration of RES. Similarly, in [11], theintegration of ESSs in a stochastic UC model reduces the wind power curtailment andthe system operation cost under a high penetration of wind power.