A power balance-dependent operating parameter and a surroundings parameter of a first energy source, such as a renewable energy source, are provided. A variable target operating parameter for the first energy source is ascertained dependent on the detected surroundings parameter. The target operating parameter can characterize a first energy source state in which a maximally possible output would be generated under the current given conditions of the surroundings. The operating frequency is then controlled dependent on the detected power balance dependent operating parameter and the ascertained target operating parameter such that a deviation of the operating frequency from a specified frequency is coupled to a deviation of the detected power balance-dependent operating parameter from the ascertained target operating parameter.