The electro-thermal model to assess dielectric heating under harmonic distorted voltages is based on the superposition of two voltages with variable frequencies and involves the Multiphysics coupling of electrical and thermal interfaces. The dielectric parameters of the test samples are first determined at sinusoidal voltages. Based on the measured data, the dielectric loss as well as the dissipated heat in terms of the heat source density under harmonic distorted voltages can be calculated. To verify the model, the heat source density of the test samples are also determined experimentally. The results show that the heat source density increases with higher frequencies, but decreases with enhanced temperature due to a negative temperature coefficient of the dielectric properties. Nevertheless, harmonics-induced voltage distortion has a negligible effect on the dielectric heating of the tested silicone rubber due to insignificant temperature change. The calculated results are comparable to the measured data, which indicates that the applied methods are suitable to model a harmonic distorted voltage stress. The proposed model can serve as the basis for other materials. A promising next step would be to incorporate nonlinear materials in the presented model, with the field grading silicone being a material of interest.