Potential losses and kinetic parameters in anodic biofilms of microbial fuel cell are determined with a theoretical model based on experimental data using Shewanella oneidensis. A phenomenological relation was applied to understand potential losses and kinetics at the anode's bio-interphase. This function was used to model theoretical current-potential curves (Nernstian j-E curves) for mediated electron transfer (MET) and direct electron transfer (DET) between 5 and 40 °C. The numerically determined potential losses were ηMET = 34–73 mV and ηDET = 93–170 mV with specific currents from 1.63 to 2.55 μA/cm2. The quantified potential losses were between 44 and 68 % of Gibbs free energy. The bioelectrode kinetics, using Tafel analysis, provided exchange currents, transfer coefficients and other parameters. An activation energy analysis showed low values for MET, Eap = 4.1 kJ/mol and DET, Eap = 9.3 kJ/mol supporting the idea of facile kinetics. Reaction rates of the DET depended more on temperature than with MET. Both showed sluggish but favourable extracellular electron transfer with Shewanella oneidensis. The modelling is proposed as a tool to determine overpotentials and quantify free energy losses in bioelectric systems. This newly developed method distinguishes itself from the more traditional Nernstian model and appears useful in basic and applied work.