Depends on the universe.
Let's use your parenthetical as an excuse to keep charge vanishingly small, because we can avoid thinking "Which charge? Which charge carrier or carriers? What's the distribution of charges?", and largely ignore the electro- effects of electrovacuum (which answers these, but in surprising ways when you look deeply).
In a chargeless vacuum Schwarzschild or Kerr universe, we have total coordinate freedom because there is nothing there but the mass at an infintesimally small point, p. If one builds a system of coordinates with p always at a single spacelike point (say, the spacelike origin), then the symmetries of this vacuum system let one chop away spatial position (e.g. const.coord.x, const.coord.y, const.coord.z, t -> 0,0,0,t) and consequently the vector-quantity linear momenta of the black hole vanish. Moreover, these vacuum spacetimes are also eternal (the black holes do not grow or shrink), so we can do t = const. too. With suitable coordinates, gives us two free parameters: mass & spin (and in Schwarzschild, just one: mass).
These solutions do not superpose additively. By the Raychaudhuri focusing theorem, if we add a point mass to the Schwarzschild black hole universe, the two masses will eventually collide. We have broken the spherical symmetry of the Schwarzschild solution, and when we solve the geodesic equations, we find caustics, where our two infinitesimal masses can be in the same infinitesimal space. We have also broken time symmetry: at past time the two masses are spacelike separated. In the future they are not, as they will merge into one black hole. We also have the problem that the black holes move with respect to one another, so we either adapt or system of coordinates to be comoving with the black holes, or we have one or both of them move against the spatial-coordinate part of our system of coordinates.
When we take this further by breaking other symmetries than the time one, e.g. by adding angular momentum to the system, we have to consider the evolution of orbital angular momentum of the pair, and possibly the spin angular momentum of each. Either of our previous-paragraph choices with respect to encoding the coordinate evolution of the spatial distance between the pair of black holes complicates the calculation of the related vector quantities.
We're still in the land of a small number of parameters, but have gone from the three time-independent [mass, spin, charge] to eleven time-dependent [mass, spin, charge, 4-location, 4-linear momentum].
We can explode the number of parameters though by returning to "what is charge?", and equipping these universes with fields of matter. At this point one runs right into the question of: "does the no-hair conjecture hold in a physically plausible universe surrounding a theoretical black hole?" or almost equivalently "when do theoretical black holes fail to approximate astrophysical black holes?", and Ligo/Virgo are good laboratories for studying whether merging black holes go completely bald.
(The hair that is supposed to bald away may be soft and indirect: the circulation of gas and dust at a distance may reveal that a given black hole was previously more than one black hole. After merger, none of that should make a difference to anything (including a 3rd black hole) falling into the balded merged black hole that the merged black hole was previously two black holes. More critically, in the enormously distant future, the evaporation of the merged black hole should not reveal the number or types of objects that fell into it during its history, whether those are black holes or some neutral mix of standard model particles. The Hawking radiation spectrum at any moment should depend only on the eleven parameters in the previous paragraph. But maybe that still-outside dust and gas has memory that remains relevant arbitrarily far into the future. Or maybe classical general relativity is wrong and rather than being crushed into a memoryless ultramicroscopic point, the ingested gas, dust and other black holes retain or at least reveal their individual identities even during evaporation).
Questions like these make black holes extremely interesting, I think.
Crane and Westmoreland looked into this: "Are Black Hole Starships Possible" (2009) https://arxiv.org/abs/0908.1803v1 https://arxiv.org/abs/0908.1803v1
A black hole is generated when mass-energy density is above a certain threshold. You need to pack about a megatonne of mass-energy in a sphere 3 attometers in diameter to get a black hole approaching a useful lifespan. You could do it with bosonic matter, but photons are easier. Well, "easier":
>In Section V below, we discuss the plausibility of creating SBHs with a
very large spherically converging gamma ray laser. A radius of 1 attometer
corresponds to the wavelength of a gamma ray with an energy of about 1.24
TeV. Since the wavelength of the Hawking radiation is 8π^2
times the radius of the BH, the Hawking temperature of a BH with this radius is on the order of
16 GeV, within the limit of what we could hope to achieve technologically.
>[...]
>In a previous paper by the first author [9], it was proposed that a SBH could
be artificially created by firing a huge number of gamma rays from a spherically
converging laser. The idea is to pack so much energy into such a small space
that a BH will form. An advantage of using photons is that, since they are
bosons, there is no Pauli exclusion principle to worry about. Although a laser
powered black hole generator presents huge engineering challenges, the concept
appears to be physically sound according to classical general relativity.
The process of aligning the laser array will be interesting, however. From Wikipedia:
>Unlike most objects, a black hole's temperature increases as it radiates away mass. The rate of temperature increase is exponential, with the most likely endpoint being the dissolution of the black hole in a violent burst of gamma rays. A complete description of this dissolution requires a model of quantum gravity, however, as it occurs when the black hole's mass approaches 1 Planck mass, when its radius will also approach two Planck lengths.
See table 2 from the paper. A 0.32 attometer wide black hole has a surface temperature of 98.1GeV and is losing 61.4 kilograms per second-- 5,519 petawatts of ultra-hard gamma radiation! Even at that diameter it has a sizable weight of 108,000 tonnes, but it doesn't have long to live-- a couple weeks. A poorly collimated laser array will produce undersize black holes which will rapidly evaporate. The paper suggests keeping them well away from Earth, on the other side of the Sun if possible. Not hard-- you'd want to put the array well within the orbit of Mercury for better solar power flux.
This is great, thanks! I hadn't thought about the Pauli exclusion principle problem, which I guess tells you how well I'd understood the problem. And of course I don't know anything about quantum gravity. I was thinking that, for use as a power plant, you want much larger black holes, not only bigger than a proton, but actually bigger than an atom, on the order of a nanometer, for two reasons: first, so that the gamma rays coming out are reasonably low energy (hundreds or thousands of electron volts instead of GeV); second, so that you can continue feeding it with regular baryons instead of keeping your spherically converging laser focused on it. But if you do that, you only get a tiny fraction of a watt out of it, and it's uselessly inefficient; even if you made trillions of these little black holes, the energy payback time of the project is measured in octillions of years if you're feeding useful photons into them.
So the sub-attometer-sized black holes described here seem like they might be a lot more practical than what I was thinking of. But... what does it take to stop TeV gamma rays? (And I guess they might also spew out strange matter and naked truth and the like, but you could probably just toss it back in.)
The billion-tonne gamma-ray laser they're talking about here is absolutely tiny.
They’d be more useful as an energy source for ships than as a power plant. You would make them in a solar system where lots of energy is available and use them to power a ship going to a different star system, one with no infrastructure of its own. Once you’ve arrived, you colonize and eventually build your own Dyson swarm to harvest the energy to make them yourself.
>don't know anything about quantum gravity
Nobody knows anything about quantum gravity, which is the problem ;)
>order of a nanometer
As db48x says, synthetic black holes aren't going to be much good as power plants, just because it's so darn hard to get them big enough that they can consume normal matter. Let's talk bare minimums. Say the nucleus of an hydrogen atom is about 2.4 femtometers wide. Plugging the numbers into https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator https://www.vttoth.com/CMS/physics-notes/311-hawking-radiati... the mass of a black hole that size is 8.07991 * 10^8 metric tons. That is a lot of laser pulse power to get into a single point just to create the black hole. Wolfram alpha claims it's 2.01718 x 10^25 watthours. The sun radiates 3.828 x 10^26 watts, so we need about a tenth of total solar output for an hour-- assuming we can convert captured solar power into coherent gamma rays with no loss, which there is currently no plausible physical way to do, so we probably need 10 or 100 times as much power. We're beyond future ultratech, and getting into solar engineering.
Assuming the black hole is created, which is hard enough, we have further problems. A 2.4 femtometer black hole is still darn hot: it's radiating a total of 545.6 megawatts of light with a peak of 51.3 MeV-- still ultrahard gamma radiation. Our beam of hydrogen ions need to fight past this headwind and into the singularity-- quickly. It's losing 6.054 micrograms per second, which will be hard to deliver as a single row of hydrogen nuclei.
Making the black hole bigger in the first place will make it easier to feed... but if you can commandeer a tenth of the Sun's output at will, you don't exactly need domestic power from a black hole.