[lmilcin]

You can also drop matter into black holes and this way recover energy necessary to sustain civilisation long after red dwarves are all dead cold.

Compared to stars, this process is much more efficient and allows converting something like 40% of mass of dropped object into energy.

And so with this mechanism we have much more energy available than all stars in the universe have and will ever have produced. Not only that, we can do it at any rate we want. Rather than radiate 99.999999999999999999999999999999999999999999% of it uselessly, we can make sure that most of it is used to power our civilisation.

The problems:

1) The visible universe shrinks, so much less matter will be available to us in trillions of years.

2) Before you drop matter into black hole you need to store it somewhere. The issue here is that over long periods of time orbits decay as gravitational energy is radiated away and so keeping mass in storage actually requires energy to be added.

3) Not gonna save us from Big Rip.

[db48x]

Only ~20% iirc. Still better than all of the alternatives, if you can build the infrastructure.

[lmilcin]

Well, I decided to check the number.

20% is for uncharged, non-rotating BH. 42% (maximum) is for rotating black hole.

I have to agree with 20%. 42% assumes that you are robbing the black hole of its momentum and that cannot be sustained infinitely, as you are throwing more and more stuff into it your best possible efficiency will drop asymptotically to 20%.