MARKAL is a generic model tailored by the input data to represent the evolution over a period of usually 40 to 50 years of a specific energy system at the national, regional, state or province, or community level. The database specifies the energy demands -industrial, commercial, residential, and transportation that need to be satisfied over the next several decades. It describes the available sources of supply of energy, either domestic resources or imports of oil, coal, natural gas, nuclear fuel, and renewable. A menu of technologies is provided for extracting, transporting, converting, and using energy, both existing technologies and those expected to be available within the time horizon of the model. The essential characteristics of the technologies are specified, such as their investment cost, operating and maintenance costs, service life, fuel use, efficiency, availability, output, and maximum expected market penetration. As a linear program, the model then chooses the best combination of these technologies to satisfy the projected energy demands. The basic components in a MARKAL model are specific types of energy or emission control technology. Both the supply and demand sides are integrated, so that one side responds automatically to changes in the other. The model selects that combination of technologies that minimizes total energy system cost. In the end, the article describes energy conservation for building in Chongqing.