Nanogrid is a small-scale microgrid that consists of loads, sources and storages interfaced with power electronic converters. Conventionally, the droop control provides power sharing. To establish autonomous operation with stable voltage and reliable power quality, an energy management system (EMS) in the form of a bidirectional multiport converter with energy storage at the PCC is used. Unlike typical approach, this paper proposes an EMS concept for nanogrids based on a single grid-forming voltage source. This allows using a simple voltage control algorithm instead of the droop control; in addition, it is compatible with all conventional power electronic interfaces. The EMS concept, its operation modes and control strategies are described. Some case studies are presented and analyzed by computer simulations.