Energy harvesting is one possibility to power up small sensor devices using ultra low power technologies. The work of this paper is based on electrostatic energy harvesters using a variable capacitor as a charge pump to convert mechanical into electrical energy [1-3, 6]. These capacitors can be implemented as discrete devices or as micro electromechanical systems (MEMS) integrated on a chip. The aim of this construction is to show a method to characterize the harvested energy of the intrinsic harvester. Due to low currents in the range of microamperes it is difficult to measure exactly without influence on the harvesting process. A new improved topology is used to ensure the autonomous operation of the harvester circuit. This topology allows for measuring makes it possible to measure the converted energy more accurately, even if there are resistive losses in the variable capacitor. Thus we can obtain comparable results on the efficiency of the harvester itself. The amount of harvested energy can be determined easily by processing the measured values. Finally this new measurement setup was implemented as a stand alone measurement device . To demonstrate the new method different types of harvesters were characterized. The electrical efficiency of the harvester output was discussed and a strategy for its optimization was developed.