Enhanced RFID tag technology especially in the UHF frequency range provides extended functionality like high operating range and sensing and monitoring capabilities. Such functionality requiring extended system structures including data acquisition units, real time clocks and active transmitters causes a high energy consumption of the tag and requires an on board energy store (battery). As a key parameter of the reliability of an RFID system is the lifetime, the energy budget of the higher class tag has to be as balanced as possible. This can be achieved by using energy harvesting devices as additional power supply. The PowerTag1 project and thus this paper proposes special energy storage structures interfacing energy harvesting devices and dealing with their special requirements for the use with battery-driven higher class UHF RFID tags. Different implementation variants of such structures are compared by using accurate simulation models of the various parts of the system. The results of the simulations are compared to manufacturer given and guaranteed system performance parameters of a state-of-the-art higher class UHF RFID system.