Transcranial alternating current stimulation modeling is a common procedure to either predict the stimulation clinical effect or to design protocols with optimal parameters. Knowledge of dielectric properties of tissues, especially conductivity, is required to perform such modeling as prior information. However, the low-frequency values of dielectric properties of human tissues are still not well established, and vary between individuals. To address this, analysis of electric field variability due to conductivity variability was assessed recently in the literature. To date, no such analysis has been performed by including permittivity (or tissue capacity) and its own variability. The present study aims to fill this knowledge gap, test the hypothesis, and quantify whether the contribution of permittivity in the analysis of dielectric properties variability impacts the resulting variability of electric field estimation. Furthermore, we provide margins for the electric field and its focality using the extreme values of dielectric properties values reported in the literature. Our results suggest that electric field magnitude, and the component normal to the cortex, are sensitive to conductivity changes, but also to brain tissues permittivity, with an error of neglecting permittivity that can reach almost 40%. Overall, these results contribute to a better understanding of tACS computational modeling.