This paper proposes an optimal power flow (OPF) framework in presence of grid scale energy storage system(s) in electric grids. It evaluates the impact of centralized storage versus distributed storage in OPF framework. Proposed OPF formulation includes a detail storage model to asses it's loss of life and associated cost due to frequent charging and discharging. Storage devices are optimally used to minimize the overall cost of generation. A meta-heuristic optimization technique called Particle Swarm Optimization (PSO) is used to solve this nonlinear and non-convex optimization problem. Proposed method has been tested on a 30-bus IEEE benchmark system and the advantage of grid scale storage has been illustrated and validated.