The topic being covered in this paper is how the energy performance certificate of a building is an indicator of the actual energy status and the report on the basis of which, by analyzing and taking certain steps, we can improve the energy efficiency of the building and achieve cost savings for the energy required without reducing the quality of life in the building itself. The introduction gives a brief overview of the historical events that have led to global awareness of the impact and consequences of our civilization's development, and recognizing that climate change is nature signalling to us that something must change and that this situation is not sustainable in the long term. Through the energetics and physics of the building we move on to the topic of the energy performance certificate itself, and a concrete example shows the process of creating one certificate for a residential single-family home with a simple technical system. By reviewing and analyzing the data obtained, we will gain insight into the critical locations where the greatest heat losses occur and, through a computer program, simulate some procedures for reducing these losses. The “Ki Expert plus” program issued by the company Knauf insulation will be used to develop the certificate and the energy efficiency budget. The program is available for people who want to carry out the energy efficiency of a building, from the company website, and can be used both to produce an energy performance certificate and to design a new energy independent facility. The program uses the latest budget regulations and is approved by the Ministry of Construction and Physical Planning and as such is approprite for this paper.