AFAIK a black hole could be a near perfect mass energy converter, and I think the evaporation rate (Hawking temperature) can be regulated by adjusting black hole spin.
There’s just the wee problem of getting or making a black hole and then grabbing it and controlling its spin.
There's the second problem that the temperature and power are directly related, so if you want the temperature low enough to not photo-ionise stuff much (6eV/70,000 K) then be power output is 114 microwatts.
There’s just the wee problem of getting or making a black hole and then grabbing it and controlling its spin.