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. 2600 m(2) g(-1). The maximum energy density of an imaginary double-layer 'nano-capacitor', is close to 10 U kg(-1) at a voltage of U = 1 V (aqueous electrolyte) of ca. 40-45 U kg(-1) at U approximate to 2.3-2.5 V (organic electrolytes), and at the order of 100 U kg(-1) at voltages close to 4 V (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. 20-30 F g(-1)), is much smaller than the corresponding value expressed versus the mass of the carbon material (ca. 300 F g(-1)). In order to obtain the energy density of the device at a level of 100 U kg(-1) (characteristic for the lead-acid battery), the capacitor with porous carbon electrodes should operate at voltages of ca. 4 V (ionic liquids as electrolytes). However, the specific power density of such a capacitor having an acceptable energy density (ca. 100 U kg(-1)) is relatively low (ca. 1 kW kg(-1)). (C) 2007 Elsevier B.V. All rights reserved.