This paper presents a decentralized approach to certifying closed-loop passivity for a class of switched graph-based dynamic models. The switched modeling framework is particularly suited to power flow systems in which paths of power flow are switched on and off. Passivity is shown to be preserved under the interconnection of multiple such graph-based models, allowing for the formation of passive 'systems of systems.' Decentralized Model Predictive Controllers paired with each system can then be formulated with a passivity-preserving constraint to ensure closed-loop stability. This allows complex energy systems to be stabilized with decentralized or distributed control architectures, while centralized control may not be practical due to the inherent computational complexity or communication bandwidth limitations. A numerical example demonstrates the efficacy of the proposed approach on a fluid tank system.