To provide ancillary services in HVDC applications, modular multilevel converters (MMCs) with integration of energy storage systems are a promising solution as they take advantage of the modularity and the controllability of the stored energy. In these solutions, an energy storage system is connected to the DC capacitor of a submodule (SM) to make an energy storage submodule (ES-SM). An MMC with partial integration (MMC-PIES) is an MMC with each arm made of a mix of SMs and ES-SMs. In this paper, we propose a novel design methodology for these converters considering they are built based on existing prebuilt submodules, while design methodologies in the literature consider the SM and ES-SM characteristics to be degrees of freedom. Therefore, the proposed approach is closer to an industrial standpoint and computes the minimum number of ES-SMs to comply with requirements. We also include a new optimization method for the circulating currents needed to balance the energy in the SM and ES-SM capacitors. Design scenarios are presented. The results show that the value of the DC capacitance and the current limitation of the switches highly influence the design, restricting the possible operating points. In addition, half-bridge ES-SMs seem to be a more promising solution than full-bridge ES-SMs, reducing the number of ES-SMs.