Exergy analysis of a 500 MWe steam turbine cycle of an operating power plant is conducted under the design and off-design conditions with different degrees of superheat and reheat sprays. The analysis shows how a first law-based analysis shows an apparent (false) improvement in a feed water heater under an off-design condition, while the actual performance degradation is reflected through an exergy analysis. The analysis also helps identifying the contribution of individual equipment in the overall increase of exergy destruction under off-design condition. Exergy analysis is also performed using off-line performance guarantee (PG) tests conducted before and after a unit overhauling. Pre-overhauling exergy efficiency figures of the major cycle equipment are compared with their respective design values to assess the need and extent of maintenance work, whereas post-overhaul exergy data is used to quantify the compliance with the guaranteed performance. Results of the study provide a quantifiable basis for formulating exergy-economy driven maintenance scheduling and PG test procedures. (C) 2010 Elsevier Ltd. All rights reserved.