  We present results from a leptonic emission code that models the spectral
energy distribution of a pulsar wind nebula by solving a Fokker-Planck-type
transport equation and calculating inverse Compton and synchrotron
emissivities. We have created this time-dependent, multi-zone model to
investigate changes in the particle spectrum as they traverse the pulsar wind
nebula, by considering a time and spatially-dependent B-field,
spatially-dependent bulk particle speed implying convection and adiabatic
losses, diffusion, as well as radiative losses. Our code predicts the radiation
spectrum at different positions in the nebula, yielding the surface brightness
versus radius and the nebular size as function of energy. We compare our new
model against more basic models using the observed spectrum of pulsar wind
nebula G0.9+0.1, incorporating data from H.E.S.S. as well as radio and X-ray
experiments. We show that simultaneously fitting the spectral energy
distribution and the energy-dependent source size leads to more stringent
constraints on several model parameters.
