Utility power producers are being driven by the inherently high efficiency and attractive installed cost of Combined Cycle Power Plants (CCPP) to consider them as the dominant choice for least-cost power. However, in a CCPP the power capability is significantly affected by the ambient temperature [Sue DC, Chuang CC, Lin PH. Performance improvement for gas turbine combined cycle power plants (GTCCPP) in Taiwan. In: Johnson D, editor. Electric power. Fourth annual conference and exhibition, vol. 4A; 2004. St Louis, MI, USA: American Center. p. 1-15. [1]], condenser pressure and power demand. In the Power Purchase Agreements (PPA) written in Taiwan, the capacity payment is based on the guaranteed power output during the operating period. Therefore, operating the CCPP at the highest performance and maintaining the guaranteed or contractual power output are important factors to get the full capacity and energy payments from the power purchaser and reduce the fuel! and operating costs for the power producer. © 2004 Elsevier Ltd. All rights reserved.