In my diploma thesis, I will present the structure of the uninterruptible energy supply system of a building and the role of its constituent functional units, its properties and construction possibilities. In addition to providing electrical energy constantly to the electronic supplies in all areas of life, the quality of the energy supply is important in many cases, too. Therefore I will also treat the topics of lightening and overvoltage protection and I will examine the questions of Electro Magnetic Compatibility (EMC). Choosing a UPS that fits into the electric network of a building or office building (in this case) and that fulfils the technical requirements is a complex task, as there are multiple types of Uninterruptible Power Supplies (UPS). Therefore, it is indispensable to examine them appropriately before the planning phase. I will present the different types of UPS and their properties, including the advantages and disadvantages that can be observed amongst them. Thereafter I will propose a solution for a specific facility. The aim of this solution is to provide uninterrupted energy supply, as well as the related protection from overvoltage and the minimisation of probable EMC effects.