So, in most sci-fi worldbuilding, there inevitably is a need for powerful energy sources to do all the magic, which simple energy sources such as burning coals, energy from the earth and possibly even Nuclear Fusion is not enough.
Thus, I ask, what are the most powerful theoretical energy sources?
Here are my notes:
No need to be present in modern times, can be in the future and theoretical, as long as it is still "hard" in nature.
Calculations are optional, but helpful.
Assume that any resources necessary to make such energy sources is there. In other words, virtually unlimited resources to build such a device.
Would like unusual or uncommon sources of energy. I would not be happy to see answers like antimatter or ZPE.
Any size for the power source is fine. From portable to stellar. Whatever size you want.