Conventionally, it is stated that Class-E Power Amplifiers provide 100% efficiency because there is no overlapping between the current and the voltage time-domain waveforms, i.e., zero device power dissipation. A further frequency-domain analysis establishes that the condition for 100% efficiency requires the previous time-domain condition plus the orthogonality of the current and voltage harmonic components higher than the first harmonic. This paper presents this harmonic analysis for the typical class E PA with linear output capacitance and extends the study to the class E PA with non-linear output capacitance. According to our results, the latter case does not fulfill the orthogonality condition and therefore it cannot be established that DC power is equal to the fundamental output power as it is assumed in other research works. Copyright 2006 IEICE.