This paper presents a simple analytical method to optimize the efficiency of an electrostatic out-of-plane gap-closing (OPGC) Vibration Energy Harvesters (VEH). For the first time the electrical and mechanical behaviours of the transducer are addressed simultaneously, while a voltage limitation on the transducer's terminals is set to prevent any damage in the conditioning electronic. The presented work allows to the designer to determine the best strategy depending on whereas the system is passive or able to be self-adapted to the external vibrations parameters. The optimization is performed for a 1 cm 2 device made of bulk silicon to be fabricated using a batch MEMS process. Up to 15 mu W can be obtained at 200 Hz if the output voltage is limited to 60 V. The calculations are validated by VHDL-AMS/ELDO simulations.