The Efficiency of Using Accretion Power of Kerr Black Holes
  The efficiency of a rapidly spinning Kerr black hole to turn accretion power into observable power can attain 32 percent for the photon emission from the disk, as is well known, following the work of Novikov-Page-Thorne. But many accretion disks are now understood to be underluminous ($L<L_{Edd}$), while still putting large amounts of energy into the jet. In this case, the apparent efficiency of jets driven by the innermost accretion disk of a highly rotating Kerr black hole ($a_*=0.999999$) can reach 96 percent. 
Ioana Dutan (1,2), Peter Biermann (2,3) (1-University of Bucharest;   2-Max-Planck-Institute for Radioastronomy; 3- Universi...