Abstract: The power level of nuclear power units is directly linked to plant economics. As the number of fuel cycles increases during unit operation, various factors can contribute to a decline in electrical power output, with some units even experiencing abnormal output issues during their first fuel cycle of operation. This paper presents two practical cases involving million-kilowatt-class pressurized water reactor (PWR) turbine generator units, where thermal power anomalies led to discrepancies in electrical power output under identical operating conditions. The findings offer valuable insights for future performance diagnostics of unit output issues.