Apparatuses, methods and storage medium associated with deriving power output from an energy harvester are disclosed herein. In embodiments, an apparatus may include one or more processors, devices, and/or circuitry to identify a plurality of times at which an intermediate voltage of a two stage power conversion circuit corresponds to a voltage reference, and ascertain an amount of time between one of the identified times and another one of the identified times. The one or more processors, devices, and/or circuitry may derive a power or current value associated with the second power supply using the amount of time.