Energy is omnipresent, in the form of the most various demands, which can be satisfied from diverse resources, through a myriad of transformers allowing extraction, transport, storage and adaptation to the desired use. During this conference we will present the basic concepts and challenges for understanding the energy system as a whole, with a focus on the domain of buildings (which represents a good 40% of the demand of our societies). Within this framework, we will discuss the challenges related to renewable energies (decarbonization) as well as the decrease in demand (efficiency, sobriety).