In the Kardashev scale, where can we put the Borg? With their advanced assimilation, adaptation, transwarp hu techs, able to build megastructures like Unicomplex and the interlinked Collective hive mind, can we say something in between a strong Type II and slightly Type III early stage?
https://en.wikipedia.org/wiki/Kardashev_scale Easily Type II. Type I, given the current inefficiency of solar panels as related to the energy (and pollution) needed for their construction, is well behind the Borg and there's no doubt the Borg found a means to make such panels a lot cleaner and efficient, or developed biotechnology to get around the issues. At least they could use such panels as, perhaps, a tertiary power source, if close enough to a star, without needing to get around trees and stormy systems. And just eject them into space or a star once their lifespan is used up. Of course, energy and other issues when powering down and up their main systems also need to be taken into account, along with other factors... Hard to say much else given their societal structure (not just energy) as TNG movies and VOY often altered their premise, to varying degrees of intrigue, as needed. Except it's open-ended enough that any possible explanation would fit in perfectly. It's the downside to using the antithesis of what amounts to "sledgehammer tactics".
Given what we've been told about the Borg it's very possible that they didn't build the unicomplex but instead took it over from the people who did, the same iwth the transwarp network. I wouldn't be surprise if the cube ships were a creation of another species that the Borg assimilated. One, unless there's a zero.