This paper analyses the impact of operating modes and nonideal power supply conditions on the efficiency of modern low-voltage power electronic devices. The sophisticated circuits and controls implemented in such devices are expected to result in increased efficiencies, higher operating power factors, and reduced harmonic emissions. However, the interactions of individual PE devices with the supplying network will impact exchanges of powers at fundamental system frequency and nonfundamental (i.e., harmonic) frequencies. This paper correlates the obtained results for harmonic performance and efficiencies over the entire range of operating powers of the considered PE devices using both standard definitions and some alternative interpretations.