Systems for process-operator guidance are now found in all industrial branches. Their impact on process performance, particularly energy use, is discussed. A case study includes an evaluation of a system in a steel plant; energy efficiency of the process was about 4% higher when the system was in use. A proposed extension of the system includes calculation of NOx emissions and emission charges. The expected process performance for simulated operating conditions is studied. Four performance criteria are used: operating cost, NOx emissions, energy and exergy efficiencies. Measurement accuracy, process modelling, and the emission-charge scheme are found to be critical for the guidance given by the system and hence the performance of the process.