To address continuously varying operating conditions in a power system with intermittent energy sources, the system should demonstrate certain level of flexibility. This paper identifies the parameters affecting flexibility in a power system and proposes an optimization model to study various impacts of these parameters. In this regard, storage, as one solution to increase flexibility, is being installed in FLEXITRANSTORE project, funded by European Commission under Horizon 2020 research and innovation programme. The model incorporates electric energy storage, variable energy resources and their curtailment, active demand response and load shedding scheme as well as operational reserve and market. Moreover, system limits such as voltage angle and transmission constraints are considered in obtaining the optimal system-oriented solutions. Additionally, a novel flexibility index (FLEXIN) is designed to translate the benefits from deploying various products for the system and society. The model is studied under different scenarios and tested on a modified IEEE 24-bus test system. The results show the advantages of the model and the index with respect to current state-of-the-art models as well as its capability in addressing current challenges of providing enough flexibility.