The paper examines the variability of key parameters in the operation of ten thermal power plants in various commercial grid environments with a view to assessing the viability of ‘on-demand’ plant performance monitoring for heat rate declaration. The plants of various types are limited to coal- and oil-fired units in the capacity range of 305–690 MW generated output. The paper illustrates the influence of control system configuration on effective and flexible power plant management. The analysis of variability indicates that there is a reasonable probability of achieving adequately stable operating periods within the normal operating envelope of grid dispatch instructions when thermal performance monitoring and display can be undertaken with a high confidence level. The levels of variability in fuel quality, which were measured during nominally constant levels of fuel input and generated output, range from about +1 per cent for oil-fired plants to about ±5 per cent for coal-fired power plants. The implications of adopting on-line monitoring of unit heat rate as an input to the generation ordering and unit commitment process are potentially significant cost and energy conservation benefits for utilities having a high proportion of coal- and oil-fired generation.