Nowadays, renewables are the first choice option for a modern power system generation scenario. It is due to their high attraction, especially environmental attraction, cost aspects and also availability in almost all over the world. Wind and solar sources are now competitive with conventional sources and command a high percentage of Investments in renewable power. The main challenge of using these cheap and clean energies is their output power uncertainty, and their variability may lead to wind/solar power curtailment, or load shedding caused by insufficient spinning and fast reserve. Energy storage systems integrated with renewable energies are a common solution for this challenge. However, they impose extra cost to planning, and operation costs need a suitable economic study for the best location and size of these systems. In this paper, a flexibility based approach is used to show the role of Battery Energy Storage System (BESS) in the wind/load curtailment reduction. This approach can lead to a suitable economic routine to determine BESS size based on economic trade-off between BESS fixed, variable costs and wind/load curtailment costs. First, the BESS flexibility index is introduced and the suitable State of Charge (SoC) control is presented to use for Dynamic Economic Load Dispatch (DELD) solution based on the wind/load curtailment reduction. The simulation results show the efficient dependency between system flexibility improved by BESS integration, and the wind/load curtailment reduction.