  We present here a new determination of the ratio of energies radiated by
active galactic nuclei and by stars and discuss the reasons for the apparently
conflicting results found in previous studies. We conclude that the energy
radiated by accretion processes onto super massive black holes is about 1 to 5%
of the energy radiated by stars. We also estimate that the total mass accreted
on average by a super massive black hole at the centre of a typical 10^11 Msol
galaxy is of about 7 10^7 Msol.
