Or it might just be pretty straightforward: Use Solar panels to harvest the energy of the sun, that is there for free goddammit.

What's the most efficient design for an energy generation device that fits in an area smaller than 1 cubic mile, uses materials found on Earth or nearby planets/passing comets, and if it outputs particle, waves, or any combination thereof that destroy the atmosphere or DNA/life, contains the destroyers?

But if we're dealing with a demon or a genie or aliens, we won't like the answer. Fuckin' tricksters.

Yeah, a Dyson sphere is arguably pretty optimal (or one of the even more outlandish interstellar level theoretical solutions). Why deal with fuel when stars are already there? There's even a classification system for how advanced a species is, which measures efficiency of energy consumption. In that, harnessing the power of a single star is a type 2 civilization (humans are generally considered on the scale of type 0.7).

But yeah, knowing that maybe it's theoretically possible does nothing to help us actually make such a thing. Even if we were to also be told exactly what materials it would take and an exact blueprint of what to do, the scale of construction is pretty much beyond current human levels.

Or maybe impossible or impractical like fusion.