This paper points out the relation between population growth and energy justice in a finite world. Reflecting on the meaning of limits, I propose to look at energy justice in a broader sense: restoring a safe and more equal operating space for humanity by stressing renewable energy concrete possibilities. Firstly, I show that during the last two centuries humanity has turned fossil fuels into human biomass. I suggest that, a new approach to energy has brought the global population to an exponential explosion. Indeed, the phenomenon called Great Acceleration is characterized by an extraordinary increase in both population and consumption. Then, I explain why a growing demographic trend can be an effective obstacle to solve the problem of energy distribution. In this direction, I explore the current lack of energy independence of most of the states outside the Organisation for Economic Co-operation and Development. Hereafter, I explain, in physical terms, why planet Earth is not an isolated system nor a closed system but a finite one. Thus, a never-ending growth in term of energy consumption is bio-physically impossible. Finally, I emphasize how a realistic perspective on energy ethics has to be weaved with an informed consideration of demographic ethics.