A method is provided for operating a gas turbine, which especially feeds power to a local isolated power supply network, and which comprises a compressor for compressing combustion air which is drawn in from the environment, a combustion chamber for combusting supplied fuel by the compressed combustion air, a turbine which is driven by the hot gas from the combustion chamber, and a generator, which is driven by the turbine, for generating electric power. With such a method, an improvement of controlling is achieved by one or more parameters of the gas turbine being measured or determined, by the effective thermal output power of the gas turbine being calculated from the measured or determined parameters, and by the calculated effective thermal output power being used for controlling the gas turbine.