  Black holes release energy via the production of photons in their accretion
discs but also via the acceleration of jets. We investigate the relative
importance of these two paths over cosmic time by determining the mechanical
luminosity function (LF) of radio sources and by comparing it to a previous
determination of the bolometric LF of active galactic nuclei (AGN) from X-ray,
optical and infrared observations. The mechanical LF of radio sources is
computed in two steps: the determination of the mechanical luminosity as a
function of the radio luminosity and its convolution with the radio LF of radio
sources. Even with the large uncertainty deriving from the former, we can
conclude that the contribution of jets is unlikely to be much larger than 10%
of the AGN energy budget at any cosmic epoch.
