  We estimate the power of relativistic, extragalactic jets by modelling the
spectral energy distribution of a large number of blazars. We adopt a simple
one-zone, homogeneous, leptonic synchrotron and inverse Compton model, taking
into account seed photons originating both locally in the jet and externally.
The blazars under study have an often dominant high energy component, which, if
interpreted as due to inverse Compton radiation, limits the value of the
magnetic field within the emission region. As a consequence, the corresponding
Poynting flux cannot be energetically dominant. Also the bulk kinetic power in
relativistic leptons is often smaller than the dissipated luminosity. This
suggests that the typical jet should comprise an energetically dominant proton
component. If there is one proton per relativistic electrons, jets radiate
around 2-10 per cent of their power in high power blazars and 3-30 per cent in
less powerful BL Lacs.
