Class-E RF Power Amplifiers (PAs) are very power efficient under nominal operating conditions. Due to incorporating two tuned tanks, the dependency on the load impedance is however relatively large, resulting in e.g. load dependent output power, power efficiency, peak voltages and peak (and average) currents which can lead to reliability issues. This paper presents load-pull analyses for class-E RF power amplifiers from a mathematical perspective, with analyses and discussions of the effects of the most common non-idealities of class-E PAs: the limited loaded quality factor (Q loaded) of the series filter, switch on-resistance, limited quality factor of the dc-feed inductor, loadmismatch dependent switch conduction loss and the limited negative voltage excursions (due to e.g. the reverse conduction of the switch transistor for negative voltage excursions). The theoretical findings are backed up by extensive circuit simulations and loadpull measurements of a class-E power amplifier implemented in 65 nm CMOS technology. The PA provides 18.1 dBm output power and 72% efficiency at 1.4 GHz under nominal operating condition employing an off-chip matching network.