## 1 Introduction
Energy storage has the advantage to time-shift the electrical energy supply thus it acts as an ideal mechanism for moderating the consequences of fluctuating output of VREs on the power system. There are many well-known types of ESS in many works of literature varying in terms of technical and economic specification as summarized in [19]. Many studies have evaluated and demonstrated the cost-benefits of appropriate selection and application of different ESS technologies incorporation into the power system planning. The common ESS ranges from pumped hydro [20], hydrogen storage [21], BESS [22], compressed air energy storage, etc. [23]. The inclusion of demand-side management into optimal component sizing that involves energy storage (ESS) facilities is proposed in [24]. The final outcomes show that using the demand-side management (DSM) increases the system flexibility and offers an economical planning option with reduced ESS capacity requirement. There are two main categories of end-user electrical demand namely the flexible load demand (FDRs) and inflexible/static load demand. The flexible load demand (FDR) are assumed to be those appliances whose time of use can be transferred from one period to another. FDRs include heat pump, room heater, washing machines, etc. They are also referred to as the shiftable appliances because their usage can be delayed during the period of peak demand or shortage of electricity supply and activated later during the period of over-generation. Non-shiftable load demands, on the other hand, are appliances that are static in terms of the time of use, they have a fixed time of period as to when to use, such as illumination loads. DSM has recently received heightened attention in terms of flexibility provision capability using flexible demand resources (FDRs) to achieve the controllability of the customer load demand pattern [25]. In general, the FDRs provide allowances and capacity for time-shifting in terms of their energy requirements. A proper schedule of the FDRs can guarantee mitigation of the gap that exists between demand and system generation for power systems with very high renewable energy fraction as addressed in [26]. A demand-side flexibility approach using the controllability of FDRs has been developed with a detailed implementation framework for commercial and residential smart building in [27]. Demand response program (DRP) is a subset of DSM designed to influences the consumer's behaviors in terms of the time of usage of the FDR through motivations such as incentive payments and lucrative electricity prices to improve the overall system efficiency [28]. The concepts of DRP have been adequately covered in the literature with much focus on optimizing the electricity market design. The commonly featured types of DRPs found in the literature includes and not limited to; real-time pricing, day-ahead pricing, time of use, interrupted curtailable, direct load control, critical peak pricing, etc. [29]. Successful implementation of DRP should take into account the current and the forecasted future power system status to fully exploit the market flexibility [30] and captures the VREs generation uncertainties [31; 32]. However, an accurate and reliable VREs output forecasting can serve as a core and vital component of energy management systems (EMS) implementation [33]. The role of forecasting also has significant value in the implementation of pricing schemes in the power markets to decrease the rate of market volatility [34]. Hence, power forecasting plays a pivotal role in flexibility planning for integrating and addressing the uncertainty of the VREs in hybrid power systems. Accurate power forecasting provides critical information of the anticipated status (power shortages and surplus) of the power system ahead of time before the actual occurrence. Hence, a good foresight of the time-ahead generation profile provides an opportunity to plan for future uncertainties adequately and cost-effectively.