Power scales to the square of exhaust velocity while thrust linearly, so if you have a very good power to weight power source like a black hole, you can use a very high exhaust velocity and thus can get by with very little reaction mass. Which is good.
Also, you can make a spaceship out of an asteroid (or from asteroid materials) so multi WTC mass is not a problem.
I see. I was assuming we were thrusting with nothing but gamma radiation, which is the speed of light, unbeatable, but I see with such small black holes, we may end up with some matter evaporating from it. I suppose that's one more benefit to larger, colder black holes: more likely to have lightspeed radiation than massive particles.
We could go for something closer to the temperature of the sun / wavelength of green light, about 10^19kg, easy to reflect, but that's about 0.1% the mass of the moon and would only output about 3.5 microwatts. Could try to balance between an amazingly reflective mirror to EUV or shorter wavelengths, which is more physically realistic than reflecting gamma rays, and usable power output, but that's simply out of reach at the moment. I wonder if we had an atomically smooth mirror, the smallest wavelength we could reflect with efficiency, and thus the highest power black hole we could use. Maybe it makes more sense for the black hole to have much higher mass than the spaceship, but 1g acceleration will be difficult.
Looks like most atoms are about 0.1nm, so 10^18kg black hole, for 350 microwatts. Like turning on a laser pointer attached to the moon and expecting usable thrust. Still far too large and not powerful enough to be a useful power source like a a 10^8kg black hole for around a petawatt. Either we figure out how to reflect deep gamma rays or it won't happen.
Also, you can make a spaceship out of an asteroid (or from asteroid materials) so multi WTC mass is not a problem.