  We present a new model for the spectral evolution of Pulsar Wind Nebulae
inside Supernova Remnants. The model couples the long-term dynamics of these
systems, as derived in the 1-D approximation, with a 1-zone description of the
spectral evolution of the emitting plasma. Our goal is to provide a simplified
theoretical description that can be used as a tool to put constraints on
unknown properties of PWN-SNR systems: a piece of work that is preliminary to
any more accurate and sophisticated modeling. In the present paper we apply the
newly developed model to a few objects of different ages and luminosities. We
find that an injection spectrum in the form of a broken-power law gives a
satisfactory description of the emission for all the systems we consider. More
surprisingly, we also find that the intrinsic spectral break turns out to be at
a similar energy for all sources, in spite of the differences mentioned above.
We discuss the implications of our findings on the workings of pulsar
magnetospheres, pair multiplicity and on the particle acceleration mechanism(s)
that might be at work at the pulsar wind termination shock.
