A little about energy sources.
For spacecraft, they can immediately be divided into storage systems and generating systems. Storage is fairly simple: nobody has yet come up with anything better than chemical batteries. Generating systems are less simple than they may appear. Broadly, current technologies fall into three categories: direct conversion, such as solar panels; chemical conversion, such as fuel cells; and thermal conversion, such as reactors. Direct conversion presents few particular difficulties, but chemical and thermal conversion do. Chemical conversion consumes reactants and requires disposal of the reaction products. Thermal conversion requires a temperature difference, whether for a thermoelectric converter or a conventional reactor. In other words, in a vacuum you have to get rid of excess heat, which is far from easy. Even such an exotic system as a droplet sprayer near the ship's bow, with a collector for the cooled droplets at the stern, was under fairly serious development. Large panels that radiate heat mean extra mass and, in a military situation, a strong giveaway of the ship's position. So when planning your ships' designs and modules, I strongly recommend thinking through the combined effects of every technical choice.