This paper proposes a comprehensive evaluating framework that enables facility operators to optimally size and dispatch their onsite energy storage systems (ESS) that might be operated as either standalone or integrated solar-plus-storage systems. The proposed model is developed on an energy procurement-based model with a mixed-integer linear programming (MILP) format, wherein the potential energy and demand charge savings though utility bill management (UBM) and revenue streams from energy arbitrage and frequency regulation (FR) opportunities are co-optimized. PJM's pay-for-performance regulation market is considered to remunerate the facility's dedicated capacities to the FR market. To investigate the impacts of facility behavior on the evaluation of various onsite ESSs, a set of 24 facilities from different end-use sectors with different functionalities are simulated. The practicality of the proposed evaluation model is then investigated through two case studies where the impacts of various optimally-operated onsite ESSs on maximizing the potential benefits are quantified.