Wide-band-gap (WBG) power semiconductor devices are gaining an increasing interest in power circuits, as they exhibit a low specific ON-resistance (RON) while providing a high blocking voltage. The energy dissipation corresponding to resonantly charging and discharging their output capacitance (Coss), however, severely limits their performance at high switching frequencies. In this work, we demonstrate a simple approach based on a small-signal measurement, to model Coss losses in transistors. The device output capacitance is modeled by an effective Coss in series with a frequency-dependent resistance R s. The proposed method is completely based on a small-signal measurement and it directly leads to a general view of frequencydependent Coss losses in power transistors. We consider four commercial devices based on GaN and SiC, and using the proposed technique, we evaluate Coss losses. We verify the model-based prediction with thermal measurements. The precise characterization of Coss-losses proposed in this paper is essential for designing efficient high-frequency power converters.