Using a high accurate power analyzer to perform efficiency measurements is a good start. However the performance and stability of the power analyzer itself needs to be proven over time. As switching frequencies keep increasing, it is not sufficient to only calibrate a power analyzer on 50/60Hz. Depending on the frequencies present in the application, a wider power frequency range is required to ensure the accuracy of the power and efficiency measurements. The minimum required power frequency range one should check its instruments, on can be determined based on system behavior, or limited by international standards. Power calibration over a wide frequency range also enables calibration of higher harmonics. This presentation explains why a wide frequency range calibration is essential to prove power measurements on your application!
Yokogawa, Michael Rietvelt