Unmanned Aerial System (UAS) are becoming viable aerial platforms for civil oriented monitoring applications. However, in most cases the selected UAS platforms are ad-hoc vehicles which include highly heterogeneous avionics. Avionics on-board may be COSTS modules that have highly different non-standard power requirements. Additionally, the available power sources in these UAS may be fairly limited or even restricted to battery units. This paper introduces the ELectrical Manager Service (ELMS), an on-board system in charge of offering a flexible power supply architecture that supports minimal reconfiguration overhead for a wide variety of UAS missions. The ELMS Service is designed to offer a continuous monitoring of the state of the power network, and a coherent and controlled response in front of power supply contingencies. The ELMS will monitor the batteries and generator status, the power consumption of the avionics and other systems, manage the connection/disconnections of systems, and provide power availability estimations. The ELMS Service is part of an architecture designed to facilitate the execution of UAS civil missions, the USAL. The USAL is built as a set of cooperating services in a purely distributed and scalable architecture with a middleware that manages inter-service communications.