  The energy density of the Ultra High Energy Cosmic Rays (UHECR) in the
Universe is a very important parameter for the solution of the puzzle of their
origin. It defines the luminosity of the UHECR sources and thus the type of
objects they are.
  This is also of crucial importance for the design of high energy neutrino
telescopes. The current attempts to derive the source luminosity are hindered
by the small world experimental statistics.
  We show that the unknown strength and structure of the large scale cosmic
magnetic fields affect strongly the UHECR propagation history.
  The identification of the UHECR sources will bring important information on
the large scale magnetic fields.
