Energy has become the keyword relevant to the economic and social development and sustainability of all countries as well as the ecological future of the world. Knowledge-based strategic planning and decisions in energy are therefore very crucial for all. Knowledge may be defined as the information needed for good planning and intelligent decisions for any human being, from a lay man to a top man in a public or private organization up to even the country president. In the article, after some introductory remarks about energy and knowledge, knowledge -energy relation is defined. Smart cities and intelligent utilities / smart energy, Energy Engineering and Knowledge Engineering terms are also elaborated. As a conclusion, the synergy and even a symbiosis between energy and knowledge is noted and it is stated that energy engineering and knowledge engineering together with nano science and technology will be very crucial for the future of humanity.