The demand for highly efficient power electronics has led to the development of wide-bandgap (WBG) power semiconductors such as gallium nitride (GaN) and silicon carbide (SiC). These WBG devices offer superior switching characteristics compared to traditional silicon-based semiconductors, resulting in higher efficiency, higher power density, and reduced size of power electronic systems. However, the fast switching of WBG devices presents significant challenges in accurately measuring power losses during a switching transition. The widely accepted double-pulse-test (DPT) has limitations due to electrical measurement difficulties, and thus, new experimental methods are required for determining power losses in modern power semiconductors. The approach employed in this thesis to measure power losses in power semiconductors is the transient calorimetric method.