The transmembrane potential (TMP) of a cell exposed to harmonic or transient electric fields is the main parameter for a successful permeabilization of a cell. Obviously, TMP can be computed with a finite element method, but the high contrast between sizes and electromagnetic properties of the cytoplasm, the membrane, and the extra-cellular medium leads sometimes to inaccurate numerical results. Influence of various parameters on the accuracy is studied. Finally, optimal electric transient waveforms are proposed for various shapes and sizes of cells