This paper presents a method for selecting locations and sizes of battery storage systems in power systems with distributed power generation. The presented method is based on determining the coherency index (C) for each bus to quantify its contribution to the frequency of the equivalent center-of-inertia (COI) during and post a transient event. Once C is determined for each bus, the principle component analysis (PCA) is used to identify buses with consistent low values of C. Such buses are identified as locations for battery storage systems. In addition, the lowest values of C for buses with low contributions to COI frequency, are used to determine adequate sizes for battery storage systems to be connected at these buses. The performance of the principle component analysis with center-of-inertia (PCA-COI) method is evaluated for Barbados power system under different transient events. Performance results show that battery storage systems (selected using the proposed method) can effectively improve the frequency stability with minor sensitivity to the levels of distributed power generation, loading levels, and/or type or location of transient events.