Predicting the performance of energy systems in all operating conditions is one of the most important issues for reducing investment costs and optimising systems efficiency. This aspect, often neglected in the utilisation of renewable energy sources, is becoming more and more important because of the variability of the sources and because the power industry is continuously expanding to new markets characterised by different energy levels. Among the renewable energy sources, geothermal energy offers a good potential for producing electrical power with a high energy level in the short and medium term. ENEL (Italian Electric Power Company) generates electricity in two large geothermal areas in central Italy, the Larderello and the Mt Amiata fields, in which vapour dominated geothermal sources are available. Two different power plant sizes were studied and built by ENEL to exploit the above mentioned geothermal fields: the 60 MW power plant suited for the Larderello fields and the 20 MW power plant in the Mt Amiata held. In this work, the main characteristics of the 20 and 60 MW power plants are described, and the results from a simulation code for the evaluation of their design and off design performance are presented. The influence of the geothermal fluid main characteristics is also studied.