A model based on dilute-solution porous-electrode theory is proposed to describe electrochemical devices that store energy in the double layer (double-layer capacitors). Various assumptions, such as neglecting concentration polarization, potential-dependent capacitance, and micropore effects, are made in the model. For constant-current discharges, the model reduces to a resistance-capacitance (RC) series circuit model after the initial discharging transients. The RC series circuit model is seen to fit existing experimental data for the discharge times available. The specific energy for constant-current and constant-power discharges is maximized over a range of discharge times by optimizing the electrode thickness, electrode porosity, and the final voltage constrained by the cutoff voltage. This maximization allows predictions of the attainable specific energies and powers for these devices and shows the influence of the various cell properties. Energy efficiencies are found for cycles when the capacitor is discharged in a given time and then charged infinitely slowly, and when the capacitor is discharged and charged in the same amount of time. Limitations to the model are discussed.