An overpotential model for planar solid oxide fuel cells (SOFCs) is developed and applied to the numerical simulation of a stack. Charge-transfer distributions within the electrodes are approximated using an exponential function, based on which the ohmic loss and activation overpotential are evaluated. The predicted current-voltage characteristics are in good agreement with the experimental results. Moreover, the overpotentials within the cell can reproduce the results obtained from a numerical analysis where the distribution of the charge-transfer current within the electrodes is fully solved. The proposed model is expected to be useful for maintaining the accuracy of SOFC simulations when the cell components, consisting of the anode, electrolyte and cathode, are simplified into a single-layer element.