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Mechanical to electrical energy conversion employing variable capacitors is assisted by electronic circuits that can have synchronous or asynchronous architectures. The later does not require synchronization of electrical events with mechanical motion, which eliminates difficulties in gate clocking and the power consumption associated with intelligent control circuitry. However, implementation of asynchronous energy harvesting circuits with the mechanical-to-electrical converter can be detrimental to the performance of the converter when done without concurrent optimization of the mechanical device and the circuit, an aspect mainly overlooked in the literature. This paper carries out system level analysis of electrostatic micro-generators with asynchronous control and charge fly-back mechanism to optimize the useful energy generated by the harvester. Our theoretical and experimental investigations show that there is an optimum value for either the storage capacitor or cycle number for maximum scavenging of ambient energy via asynchronous electrostatic transduction. The analysis also indicates that the maximum power is extracted from the system when approaching synchronization of mechanical and electrical events. However, there is a region of interest where the storage capacitor can be optimized to produce almost 70% of the ideal power, taken as the power harvested with synchronous converters when neglecting the power consumption associated with synchronizing control circuitry. Theoretical predictions are confirmed by measurements on an asynchronous energy harvesting circuit implemented with a macro-scale electrostatic converter prototype.