The combined use of Radioisotope Thermoelectric Generators (RTGs) alongside Solar Arrays (SAs) and batteries is a promising strategy for ensuring a reliable and sustainable power supply for space missions. While SAs harness sunlight when available, RTGs provide a continuous source of power regardless of the availability of sunlight. A battery provides for the times that the transient power demands exceed the combined SA and RTG generation. This combination offers a versatile and robust solution for the energy needs of space missions, ensuring optimal operation in a variety of conditions and locations within the solar system. An efficient design, in terms of available power and mass, of the Electrical Power Subsystem (EPS) is mandatory for the success of the mission. The different power sources must be properly managed so most of the power is available for the loads whilst keeping the sources at their most convenient operating conditions. Different architectures can be used whether they are based on a power bus voltage-regulated or on a power bus whose voltage is directly defined by one of the power sources. Regarding the RTG, one of the main concerns is adapting its low output voltage to the bus voltage. This can be done in several ways. For instance, by serializing several RTGs (efficient but introduces reliability concerns), using DC/DC conversion technologies (straightforward but increases power losses), or using partial processing techniques (reduces power loss but increases complexity). This study is to analyse each architecture to see their benefits and disadvantages and assess them qualitatively and quantitatively. Every proposed architecture is evaluated by sizing each power source and the units used to regulate them, according to different mission requirements. Then the mass and the electrical power available to the loads is computed. For that purpose, models of each component (SAs, RTGs, batteries, DC/DC converters) are used to evaluate every scenario. The results obtained on this study, are represented in a series of figures of merit, related with the mass of the power sources, the efficiency of the regulators used, the energy dissipation in terms of temperature (which can affect to the RTG behaviour), the maximum operation time and the battery time recharge.