Power plants use burning processes with a heavy environmental impact. Traditional design models aim to optimize economic and thermodynamic performances. Owing to the new sensitivity arising towards the environment, power generation design must respect the environment. Today no standardized design criteria exist; the present work aims to elaborate a specific methodology for power plants consisting of a predictive instrument (the Environmental Impact Evaluation), a management tool (the Environmental Management System), and a series of environmental studies and checks. After showing the methodology, the authors tested it with an application to a real scenario: the heat and electrical power needs of a pharmaceutical firm. Cogeneration seemed to be the best available technology from an economic and environmental point of view. In the proposed case study, all possible impacts have been quantified verifying legislative standards and accomplishing the ISO 14000.