The recent worldwide effort on the environmental issue has led to new regulations on greenhouse gases emissions (GHG), both for land and marine applications. Nowadays, the extensive electrification of transportation systems is a promising choice for this purpose. In this perspective, algorithms for the optimum sizing and management of energy storage systems (ESSs) integrated into already operating shipboard power systems are proposed in this paper. The main aim of this method is to reduce the power generation system fuel oil consumption, GHGemissions, and management costs. This is applied to two case studies (i. e., a ferry and a platform supply vessel), of which load power profiles are available from the on-board integrated automation system. The results yielded showremarkable savings close to6% and32% along the whole ship's life horizon for the ferry and the platform supply vessel, respectively. These results prove that an optimal sizing combined with an optimum management of ESSs may significantly reduce the operative costs of shipboard power systems.