  Estimates on different scales of the power of relativistic bulk motion in
extragalactic and galactic jets are presented. The power in the jets and the
power produced by the accretion disk are found to be roughly equal. This may
suggest an important role of the magnetic field in extracting both the
gravitational energy of the accreting matter and the rotational energy of the
black hole. A method to derive the minimum power of jets is discussed, and used
to find lower limits to the jet power of blazars and the galactic superluminal
source GRS 1915+105. The matter content of jets is discussed, finding that
electron positron pairs cannot be dynamically important. The jet power can
initially be in the form of Poynting flux, gradually accelerating matter until
equipartition between bulk motion of the particles and Poynting flux is
reached.
