  The mass of a Kerr black hole can be separated into irreducible and
rotational components -the former is a lower limit to the energy that cannot be
possibly extracted from the event horizon and is related to its area. Here we
compute the irreducible masses of the stellar-mass black holes observed by
gravitational-wave interferometers LIGO and Virgo. Using single-event data, we
present a re-parametrization of the posterior distribution that explicitly
highlights the irreducible and rotational contributions to the total energy. We
exploit the area law to rank the black-hole mergers observed to date according
to their irreversibility, thus providing a guide to selecting events for
targeted tests of General Relativity. Using population fits, we compute the
rate by which the total area of black-hole horizons increases due to the
observable mergers.
