The invention relates to decentralized energy production. In particular, the invention concerns a system for coupling a monophase power source to an internal multiphase power network of a household, company, or other property. The internal network is further connected to an external power distribution grid. The system comprises an interface unit comprising a first connection point for said monophase power source and a second connection point for said multiphase power network, the interface unit allowing for monophase power from the monophase power source to be fed to the multiphase network, and means functionally connected to the interface unit for monitoring the loading states of individual phases of the multiphase power network. The interface has coupling means to couple monophase power to selectively one of the phases of the multiphase power network based on the loading states of the individual phases of the multiphase power network. The invention improves cost-efficient usage of locally produced power and saves transmission losses.