  The energy spectrum of ultra-high energy cosmic rays above 10$^{18}$ eV is
measured using the hybrid events collected by the Pierre Auger Observatory
between November 2005 and September 2010. The large exposure of the Observatory
allows the measurement of the main features of the energy spectrum with high
statistics. Full Monte Carlo simulations of the extensive air showers (based on
the CORSIKA code) and of the hybrid detector response are adopted here as an
independent cross check of the standard analysis (Phys. Lett. B 685, 239
(2010)). The dependence on mass composition and other systematic uncertainties
are discussed in detail and, in the full Monte Carlo approach, a region of
confidence for flux measurements is defined when all the uncertainties are
taken into account. An update is also reported of the energy spectrum obtained
by combining the hybrid spectrum and that measured using the surface detector
array.
