The main thing is, I want this to still be functional for players that are starting on a map where they need to stay on just their base planet for a while. Depending on the chosen bonus ship type, some of them cost a lot more energy than others (such as parasites). I've been erring a bit on the side of not restricting it too much, while still hopefully making it more interesting in longer games than it was before. In my current solo game, loading that up required me to have Mark II reactors on 5 out of my 16 planets (and that was with 8 on my home planet, many suddenly quite low on efficiency). So I don't think it will be a case where every planet needs a reactor, at this stage.
Also bear in mind that orbital command stations each require a large chunk of energy 3k for the basic one, much more for the higher-level ones, as do certain techs like force fields, etc. So if a player is specializing in those defensive or economic technologies, then their energy producers become a lot more interesting to try to defend, too. If you play just vanilla offensive ships (which I mostly am in my solo game), then it's less of an issue.