Embodiments of the invention relate to an input-powered comparator. Embodiments of the invention also pertain to an active diode that includes an input-powered comparator and a switch. In a specific embodiment, the input-powered comparator only consumes power when an input source provides sufficiently high voltage. Embodiments of the active diode can be used in an energy harvesting system. The comparator can be powered by the input and the system can be configured such that the comparator only consumes power when the input is ready to provide power to the load or energy storage element. In a specific embodiment, when there is no input, or the input is too low for harvesting, the comparator does not draw any power from the energy storage element (e.g., battery or capacitor) of the system.