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. Influences of membrane conductivity and frequency on the accuracy are studied. Optimization of transient waveforms is proposed for various shapes of cells.