  Galactic outflows are now commonly observed in starburst and active galactic
nuclei (AGN) host galaxies. Yet, there is no clear consensus on their physical
driving mechanism(s). We have previously shown that AGN radiative feedback,
driven by radiation pressure on dust, can account for the observed dynamics and
energetics of galactic outflows, provided that radiation trapping is taken into
account. Here we generalise our model results by explicitly considering the
temporal evolution of the central AGN luminosity, and the shell mass evolution
in different ambient density distributions. In the case of fixed-mass shells,
the high observed values of the momentum ratio ($\zeta = \dot{p}/(L/c)$) and
energy ratio ($\epsilon_k = \dot{E}_{k}/L$) may be attributed to either
radiation trapping or AGN luminosity decay. In contrast, for expanding shells
sweeping up mass from the surrounding environment, a decay in AGN luminosity
cannot account for the observed high energetics, and radiation trapping is
necessarily required. Indeed, strong radiation trapping, e.g. due to high
dust-to-gas ratios, can considerably boost the outflow energetics. We obtain a
distinct radial dependence for the outflow energetics ($\zeta(r)$,
$\epsilon_k(r)$) in the case of radiation trapping and luminosity decay, which
may help discriminate between the two scenarios. In this framework, the
recently discovered `fossil' outflows, with anomalously high values of the
energetics, may be interpreted as relics of past AGN activity. The observed
outflow properties may therefore provide useful constraints on the past history
of AGN activity and/or the physical conditions of the outflow launch region.
