  We develop a model that establishes a quantitative link between the physical
properties of molecular aggregates and their constituent building blocks. The
relation is built on the coherent potential approximation, calibrated against
exact results, and proven reliable for a wide range of parameters. It provides
a practical method to compute spectra and transfer rates in multi-chromophoric
systems from experimentally accessible monomer data. Applications to F\"orster
energy transfer reveal optimal transfer rates as functions of both the
system-bath coupling and intra-aggregate coherence.
