This paper aims to show the essential aspects of the development of the physical concept of energy, in order to subsidize the choice of teachers on the best way to understand and teach this concept, and it is divided into two parts. Firstly, we have shown some contributions by Thomas Young, Johann Bernoulli, Galileo Galilei, René Descartes and Leibniz, and how was the evolution of the physical concept of energy in the context of machines and beyond the mechanics. In the second and final part, we have finished our analysis presenting some decisive contributions of Joule and Kelvin for the correct and modern formulation of the principle of conservation of energy and, consequently, of the physical concept of energy. Then, we have given our suggestion on how to approach this concept in the classroom. [ABSTRACT FROM AUTHOR]