We present the results of our theoretical investigation on the energetics of boron-interstitial clusters in silicon. In particular, we discuss the results obtained within a Density Functional derived Tight Binding (DFTB) scheme and further analyzed by Density Functional Theory calculations performed in the Local Density Approximation (DFT-LDA). We show the lowest-energy geometries, analyzing symmetry properties and atomic coordinations. The comparison of binding energy values allows us to discuss the competition between the formation of mixed and monoatomic clusters, in the search for a growth pattern in either high or low Si self-interstitial content. An upper limit for B-clusters dimensions (with n,m similar or equal to 4) is proposed. We focus then on the characterization of the electronic structures of the B-Si clusters. A preliminary analysis of the calculated electronic properties of selected cluster structures attributes features appearing in the fundamental gap to specific bonds between B and interstitial Si atoms.