  We present a bolometric light curve model of Type IIn supernovae powered by
supernova ejecta colliding with a circumstellar medium. We estimate the
conversion efficiency of the ejecta's kinetic energy to radiation at the
reverse and forward shocks and find that a large density contrast makes a
difference in the efficiency. The emission from the reverse shock can maintain
high efficiency for a long time, and becomes important at the late phase of the
light curve. We first construct a semi-analytical model that is applicable to
the late phase of the light curve when the diffusion time of photons in the
shocked region becomes negligible. We further develop radiation transfer
simulations that incorporate these physical processes into the light curve. The
numerical calculations predict light curves at early phases, which are testable
by present and future short-cadence surveys. We compare our model with the
bolometric light curve constructed from observations for a type IIn supernova
2005ip. Due to the reduced efficiency at the forward shock, we find from our
model that the mass-loss rate of the progenitor star was $\approx 1\times
10^{-2}\ {\rm M_\odot \ yr^{-1}}$ for a wind velocity of $100\ {\rm km \
s^{-1}}$, an order of magnitude higher compared to previous work that used
simple assumptions of the efficiency. This highlights the importance of taking
these two components into account when extracting the physical parameters from
observations.
