  Among the blazars detected by the Fermi satellite, we have selected the 23
blazars that in the three months of survey had an average gamma-ray luminosity
above 1e48 erg/s. For 17 out of the 23 sources we found and analysed X-ray and
optical-UV data taken by the Swift satellite. With these data, implemented by
archival and not simultaneous data, we construct the spectral energy
distributions, and interpreted them with a simple one-zone, leptonic,
synchrotron and inverse Compton model. When possible, we also compare different
high energy states of single sources, like 0528+134 and 3C 454.3, for which
multiple good sets of multi-wavelength data are available. In our powerful
blazars the high energy emission always dominates the electromagnetic output,
and the relatively low level of the synchrotron radiation often does not hide
the accretion disk emission. We can then constrain the black hole mass and the
disk luminosity. Both are large (i.e. masses equal or greater than 1e9 solar
masses and disk luminosities above 0.1 Eddington). By modelling the non-thermal
continuum we derive the power that the jet carries in the form of bulk motion
of particles and fields. On average, the jet power is found to be slightly
larger than the disk luminosity, and proportional to the mass accretion rate.
