Cellular functioning is based on coupled biochemical processes enabling the conversion of chemicals and energy from on form to another. A notable example is metabolism, through which cells convert the energy stored in energy-rich molecules, such as sugars, into a form that is readily usable, typically ATP. A mechanistic understanding of cellular energetics would greatly enhance our understanding of cell biology, but is hindered by (i) the complexity of cellular processes (metabolism, for instance, may involve thousands of chemical reactions and species), and (ii) the fact that cells operate far from thermodynamic equilibrium, i.e. a continuous supply of energy is required. In my presentation, I will first review the nonequilibrium thermodynamics of generic networks of chemical reactions, such as metabolism. Leveraging the notion of conservation law, I will introduce an algorithmic way of identifying a minimal description of the fluxes of chemicals and energy operated by network, as well as the thermodynamic forces that drive the network far from equilibrium. I will finally show how these tools can be used to assess energy conversion in central energy metabolism, the core of cellular metabolism.