  Observational evidence suggest that shocks may affect the spatial and
velocity distributions of gas in the NLR/ENLR of some active galaxies. It thus
seemed plausible that shocks may also energize the NLR. The observed emission
line ratios strongly favor photoionization as the heating source, but it is not
clear whether the ionizing radiation is generated in the NLR by "photoionizing
shocks" or whether it originates at the central continuum source. Here I point
out that shocks are highly inefficient in producing line emission. Shocks in
the NLR can convert at most 10^{-6} of the rest mass to ionizing radiation,
compared with a maximum conversion efficiency of ~0.1 for the central continuum
source. The required mass flow rate through shocks in the NLR is thus a few
orders of magnitude higher than the mass accretion rate required to power the
NLR by the central continuum source. Since gravity appears to dominate the NLR
cloud dynamics, shocks must lead to an inflow, and the implied high inflow
rates can be ruled out in most active galaxies. NLR dynamics driven by a
thermal wind or by some jet configurations may produce the mass flux through
shocks required for photoionizing shocks to be viable, but the mass flux inward
from the NLR must be kept ~100-1000 times smaller. Photoionizing shocks are a
viable mechanism in very low luminosity active galaxies if they are highly
sub-Eddington (<~10^{-4}) and if they convert mass to radiation with a very low
efficiency (<~10^{-4}).
