This study presents a theoretical framework to model spatio-temporal heat generation rates and temperature evolution in electrochemical double layer capacitors composed of porous electrodes during constant-current cycling. Expressions for reversible and irreversible heat generation rates are derived based on porous electrode theory. Temperature predictions obtained from the model are found to match experimental trends reported in prior literature as well as quantitative results under various charge/discharge conditions. The model has been applied to investigate the influence of electrode and separator porosity on the operating temperature inside the capacitor during charge/discharge. Furthermore, scaling analysis of the electrothermal model leads to a reduced number of meaningful dimensionless parameters governing electric potential, heat generation rate and temperature rise in the capacitor. We explore the influence of these dimensionless parameters using detailed numerical simulations.