Designing the control infrastructure of future ``smart'' power grids is a challenging task. Such grids will integrate a wide variety of heterogeneous producers and consumers that are unpredictable and operate at various scales. Smart grids will need to control these in order to attain global objectives at the macro-level, while also taking into account local objectives and private interests at the micro-level. This paper proposes a holonic control architecture to help meet these requirements. We show how this architecture can integrate heterogeneous control solutions, including - when applicable - existing state-of-the-art solutions for the smart grid. To better illustrate the utility of this generic architecture we exemplify its use via a proof-of-concept implementation, integrating some basic control solutions. We show how this sample holonic controller can manage a grid simulator in several scenarios. Obtained results support our belief that the proposed architecture can facilitate the development of control solutions addressing the aforementioned challenges.