This paper describes one approach to model spacecraft power systems. Modelling of solar array, batteries and power conditioning units is achieved for the simulation and analysis of in-flight performances of satellite power systems. The programme EBLOS operates from a minicomputer and at present deals with heliosynchronous orbits. However, for a maximum of flexibility, its design has been based on two features: a modular structure copied on the natural structure of power system and a nodal description input. The outputs are given in a form compatible with real in-flight measurements. The programme is described and validated against the ERS-1 Project.