High Temperature Electrolysis (HTE) of water steam seems to be a future competitive and feasible process for massive hydrogen production with no greenhouse gas emission that could contribute to respond to the increasing hydrogen demand for industrial applications and energy systems. In order to electrolyse the water steam at high temperatures, cells produced with Solid Oxides are mostly used. As a new approach, this work aims at estimating the electric potential that should be imposed on these cells based on the evolution of their electric characteristics. Two different configurations are presented in this study, the electrolyte-supported and cathode-supported cells. They are composed of stainless steel interconnects, Ni-YSZ cathode, LSM anode and YSZ electrolyte. A numerical tool based on an electrochemistry model, including the mass and energy balances and the overpotentials estimated by the Fick’s mass transfer model, Butler-Volmer equation, Chapman-Enskog equation and Ohm’s law was developed. Taking into account the electric profile and thermal characteristics along the cell, the electric potential that should be imposed on them was estimated. Besides the results validation with the experimental and modelled data from several authors, with this novel approach we observed that an increase of the electric potential up to 10% is noted between what is presented in the literature and what is calculated in this work, based on the real conductivities of all the materials used to build the cell, interconnects, electrodes and electrolyte included.