  Aims. A simplified model of jet power from active galactic nuclei is proposed
in which the relationship between jet power and disk luminosity is discussed by
combining disk accretion with two mechanisms of extracting energy magnetically
from a black hole accretion disk, i.e., the Blandford-Payne (BP) and the
Blandford-Znajek (BZ) processes.
  Methods. By including the BP process into the conservation laws of mass,
angular momentum and energy, we derive the expressions of the BP power and disk
luminosity, and the jet power is regarded as the sum of the BZ and BP powers.
  Results. We find that the disk radiation flux and luminosity decrease because
a fraction of the accretion energy is channelled into the outflow/jet in the BP
process. It is found that the dominant cooling mode of the accretion disk is
determined mainly by how the poloidal magnetic field decreases with the
cylindrical radius of the jet. By using the parameter space we found, which
consists of the black hole spin and the self-similar index of the configuration
of the poloidal magnetic field frozen in the disk, we were able to compare the
relative importance of the following quantities related to the jet production:
(1) the BP power versus the disk luminosity, (2) the BP power versus the BZ
power, and (3) the jet power versus the disk luminosity. In addition, we fit
the jet power and broad-line region luminosity of 11 flat-spectrum radio
quasars (FSRQs) and 17 steep-spectrum radio quasars (SSRQs) based on our model.
