Where does the energy come from? A three-way test for on-board energy harvesting

Proposals for on-board vehicle energy harvesting are usually debated in terms of conversion efficiency. This note argues that efficiency is the wrong first variable: the sign of the energy balance is fixed by where the harvested energy originates. Sources are sorted into three classes: ambient (A), recovered (B) and parasitic (C). Using actuator-disc momentum theory in the vehicle frame, it is shown that a turbine driven by the vehicle's own relative wind returns at most (1 − a) of the additional propulsion power it imposes, where a is the axial induction factor; at the Betz optimum this is two-thirds, before any mechanical or electrical loss. The two conditions under which Class C hardware behaves as a Class B device are stated, and the practical consequences for solar, regenerative and small ambient sources are summarised. An earlier version of this note was published on the author's blog: https://hrohera27.blogspot.com/2026/09/on-board-energy-harvesting-three-way-test.html