Abstract: A thermal performance monitor informs the operator and result's engineer of the economic losses, efficiencies, deviation in heat rates and power losses of operating a steam turbine-generator system at its controllably selected pressure and temperature. Specifically temperature and pressure signals are generated at various points in the system along with the control valve position signal and the electric output signal from the electric generator. This data is processed along with the corresponding design values and the economic losses due to temperature deviation, pressure deviation and exhaust pressure deviation from design are calculated. Other calculations produce a comparison of efficiencies of the turbines in the system and consequential power losses.