Energy harvesting is increasingly enabling the expansion of wireless sensor networks in challenging applications by replacing batteries in low power sensors. Many forms of energy harvester suffer from low output voltage that can be partially compensated for by the use of a Cockcroft-Walton voltage doubler ahead of a dc-dc converter. Impedance matching of energy harvesters is critical to achieve high output power/unit volume. This paper explores optimum impedance match for an energy harvester with a voltage doubler and dc-dc converter. Formulas are derived and experimentally confirmed, which calculate optimum impedance match between the harvester and a load, and calculate voltage at the input to the dc-dc converter for a given wireless sensor power consumption. Further, the formula for optimum impedance match is validated against independently published results.