Two types of double-layer capacitors, based on carbon materials, were analysed: (1) an imaginary nano-capacitor assembled from single graphene sheets, separated by electrolyte layers (thickness of nanometers) and (2) a capacitor based on porous carbons. It has been shown that the maximum specific surface of a porous carbon material which may be used for the construction of a capacitor is ca. 2600m2 g−1. The maximum energy density of an imaginary double-layer ‘nano-capacitor’, is close to 10kJkg−1 at a voltage of U =1V (aqueous electrolyte) of ca. 40–45kJkg−1 at U ≈2.3–2.5V (organic electrolytes), and at the order of 100kJkg−1 at voltages close to 4V (ionic liquids as electrolytes). The real device consists of porous electrodes and a separator, both soaked with the electrolyte, as well as current collectors. Consequently, the maximum electric capacity expressed versus the mass of the device (ca. 20–30Fg−1), is much smaller than the corresponding value expressed versus the mass of the carbon material (ca. 300Fg−1). In order to obtain the energy density of the device at a level of 100kJkg−1 (characteristic for the lead-acid battery), the capacitor with porous carbon electrodes should operate at voltages of ca. 4V (ionic liquids as electrolytes). However, the specific power density of such a capacitor having an acceptable energy density (ca. 100kJkg−1) is relatively low (ca. 1kWkg−1).