Energy of descent
Consider first, a maximal black hole. A particle in orbit will lose energy due to gravitational radiation and hence the orbit will necessarily decay. When the orbit decays to that of the innermost stable circular orbit, we know that henceforth the particle has to fall ever closer to to the black hole event horizon. We also know that that the black hole cannot grow anymore since it is already at its' maximal limit. So how is this to be resolved? There are only two possibilities: the conversion of the mass to energy, and/or the emission of matter at such a speed that it can escape the gravitational pull of the black hole. For a charged particle energy release will include electromagnetic radiation, but for an uncharged particle, it must be wholly in the form of gravitational radiation. Secondly, there is the possibility of energy being lost by the polar emission of particles at very high speed, as a relativistic jet, but any mechanism for this remains unclear at present. (See 'Unresolved problems' for our latest thinking.)
For a black hole that is not yet at its' maximal limit, the same effect will lead to substantial conversion to radiation, but at somewhat lower than the 100% conversion factor. We will be investigating this more fully here in due course. (See 'Unresolved problems' for our latest thinking.)
This has particular relevance to the situation that arises when black holes meet. We will look at this next.