This paper addresses the experimental and energetic analysis of the performance of a 760 MW Combined Cycle Gas-Steam Turbine Power Plant, built in Sparanise (Italy) and went into service in September 2007. The Plant object of the study faces the problem of Frequency Regulation in the restructured Italian power system which, following the European deregulating process, has been moving from a vertically integrated structure to a new scenario, with several private hold companies competing on the generation side. Bound by electricity market price and dispatching regulations, the power plant does not operate at constant maximum load (Base Load), but it is forced to continuous load variations. The new legislation revealed a number of technical and economic issues for Combined Cycle Power Plants designed to achieve its maximum efficiency in the operation at the nominal point, thus affecting its efficiency as well as operations and maintenance costs. For the partial evaluation of these aspects, in this part of the work experimental investigations and energetic analyses of current performance have been carried out. Subsequently, the main results from the proposed research activity will be used in Part II of the article, to perform thermodynamic analyses.