  We present a statistical mechanical calculation of the thermodynamical
properties of (non rotating) isolated horizons. The introduction of Planck
scale allows for the definition of an universal horizon temperature
(independent of the mass of the black hole) and a well-defined notion of energy
(as measured by suitable local observers) proportional to the horizon area in
Planck units. The microcanonical and canonical ensembles associated with the
system are introduced. Black hole entropy and other thermodynamical quantities
can be consistently computed in both ensembles and results are in agreement
with Hawking's semiclassical analysis for all values of the Immirzi parameter.
