Supercapacitors are electrochemical double layer capacitors, dedicated for the energ storage. Even if the energy density is still lower than that one of batteries, the supercapacitors are defined with a higher power density. The main advantage of that property is the time for charging and discharging the energy that can be strongly reduced. The limitation in the reduction of that time id defined by the series resistor of the supercapacitors , wich defines the energy efficiency of the component. This has to be taken into account for the sizing of a supercapacitor tank, to identify the en energy efficiency that tank and avoid any over-sizing of the storage system. This paper presents a method for the calculation of the energy efficiency of a supercapacitive bank, together with design criterion to size a supercapacitor tank into account the energy efficiency.