Advanced monitoring techniques can play a key role in improving energy efficiency of operating energy intensive systems. In particular, thermoeconomic diagnosis aims at the determination of fuel consumption variation, the identification of causes of its increment from design conditions and the quantification of the effect of each one of these causes. A thermoeconomic diagnosis system installed in a coal-fired power plant has been used to analyse its operation during a time span of more than 6 years, quantifying the effects of variations in components (degradation, repairing and substitution), fuel quality, ambient conditions and operation strategy. The diagnosis method proposed (quantitative causality analysis) provides a precision of +/-3% in addressing the source of inefficiency for about 70% of the cases. (C) 2009 Elsevier Ltd. All rights reserved.