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Combustion turbine combined cycle (CTCC) plants have generally been the “power plant of choice” over the past two decades for a number of reasons, including first cost, efficiency, and low emissions. Combustion (Gas) turbine (CT) based plants now account for over 30% of the electric power capacity in the United States. Despite the significant reliance on this technology, the electric Independent System Operators (ISOs) have yet to recognize and acknowledge in their production templates, test forms and performance predicting software the Brayton Cycle limitations, most notably how humidity affects output for CT plants equipped with evaporative cooling systems. Such plants account for an estimated 48% of the CT power installed in the last 10 years. Ignoring the impact of humidity on these plants can lead to errors in production predictions beyond the normal tolerance band of 3% to as high as 9% during peak ambient temperatures for certain units. As such the electric ISO’s prediction of available generation and the associated capacity reserve margins have the potential to be overestimated. The article explores the situation in more depth, presents examples within the NYISO, quantifies the potential impact and recommends easy solutions to close the gap.Copyright © 2010 by Sigma Energy Solutions Inc.