This thesis explores the application of Composite System Reliability (CSR) assessment in power system planning, presenting an open-source evaluation tool developed in a dynamic programming language with Monte Carlo Simulation (MCS) and parallel computing capabilities. The tool addresses historical barriers by offering a transparent methodology and flexibility in incorporating various power flow methods and external solvers for large-scale projects. Energy storage, as a composite resource and transmission asset, presents both opportunities and challenges in power solutions. The thesis demonstrates the feasibility of calculating its equivalent transmission capability aligned with traditional alternatives, identifying optimal power and energy ratings for storage systems. While enhancing network adequacy, challenges such as capacity value saturation due to oversized systems are discussed. CSR assessments examine network changes' impact, emphasizing the necessity of informed decisions through security and economic analyses.