The historiography of the principle of energy conservation has concentrated on the formulation of the law by a few individual scientists. This paper turns to the employment of energetic considerations, examining the uses of related arguments in scientific reasoning before the formulation of a well-defined principle. It shows that conceptual ambiguity and a limited formal realm of validity did not prevent the successful employment of such notions to generate novel scientific results. From the late 1810s to the 1840s, researchers including Fresnel, Ampère, Carnot, Roget, Faraday, and Liebig invoked proto-energetic arguments to address particular problems concerning wave optics, electromagnetism, theory of electric batteries, heat motors, and animal heat. Thereby they extended the realm of applicability of arguments based on the conservation of “power” beyond non-frictional mechanical systems, where the conservation of the living forces ( vis viva ) was accepted in the early nineteenth century; they also furnished scientists a theoretical toolkit with a new powerful method. Their development of proto-energetic arguments as tools for reasoning was an important historical process in itself, which together with other developments led to the emergence of “energy physics.” This history, thus, exemplifies the important role played by practices of reasoning in the formation of scientific laws and principles.