Wide-band-gap technologies enable ultra-high efficiencies in high-frequency power conversion. However, inadequate accuracies in electrical measurements could lead to spurious efficiency measurements, even above 100%. Furthermore, bandwidth limitations, particularly for current probing, delay mismatches between current and voltage probes, loading effects and electromagnetic interferences make electrical techniques inappropriate for measuring extremely-low losses in switches with high dv/dt and di/dt values and magnetic components with high-frequency excitations. Calorimetric methods overcome this issue by a direct loss measurement through the generated heat. Nevertheless, limited ranges and accuracies of existing systems hinder their application in sensitive measurements. Moreover, time-consuming calibrations with extensive data processing impede rapid design assessments. In this work, we further investigate a previously proposed closed-type double-chamber calorimeter and present its high accuracy for the evaluation of low levels of losses in high-frequency power inductors. The system provides adjustable cooling and can measure losses as low as 500 mW, enabling evaluation of high-performance power converters and their components with no dependencies on the geometries of the evaluated devices/systems.