NOVELTY - The method involves determining expected evolution of electric power produced by a power plant (12) during a first time interval based on weather forecasting on the time interval. An electric power level and a second time interval are determined based on the expected evolution. An initial terminal and/or an end terminal of the second time interval are modified according to a desired evolution of a charge state of an energy storage system (14), where the desired evolution is determined from an energy storage system model during the first interval.    USE - Method for determining electric power provided by an electrical energy supply installation, using a renewable energy source e.g. solar energy, wind energy, wave energy, river energy, ocean current energy, and/or tidal energy (all claimed).    ADVANTAGE - The initial terminal and/or the end terminal are modified according to an desired evolution of a charge state of the energy storage system so as to determine electric power on a constant level during time intervals using the renewable energy source while providing a difference between the electric power provided by the installation and forecasted power to be lower than a specific threshold. The method enables determining the electric power using a simple program, which ensures functioning of a processor, or by a dedicated electronic circuit having a simple design.    DETAILED DESCRIPTION - The energy storage system includes a system chosen among a group of electric batteries, a momentum wheel system, a compressed-air system, a pumping energy transfer station and a fuel cell and electrolyser system. INDEPENDENT CLAIMS are also included for the following:    (1) a method for supplying energy    (2) a computing medium having a set of instructions for determining electric power provided by an electrical energy supply installation    (3) an electrical energy supply installation.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of an electrical energy supply installation.    Electrical energy supply installation (10)    Power plant (12)    Energy storage system (14)    Interface system (16)    Electrical network (18)