The present research was conducted in the context of my dissertation defense. In the present study, an optimization model is developed in GAMS. It is a mathematical program, that minimizes the total cost of a power supplier, in order to meet its demand. In the introductory chapter lies a brief introduction of the modern energy sector. In particular, the energy sectors of the developed and developing world economies are presented and compared. Through this comparison, the noticeable differences between the two are distinguished, and the typical examples of China, the European Union and the United States are presented. The second chapter presents the structure and operation of an electrical system, as well as its division into two main parts. The first is an electrical system that consists of producing, transporting and distributing electricity to the final consumer. Therefore, these individual parts are analyzed and presented, giving importance to the special need of electricity, in relation to other products, which is the constant balancing of production and demand. There is also a reference to the gradual load reduction, and the pre-existing and modern methods of electricity storage. The other main part of an electricity system is the electricity market and its whole structure. Therefore the organizational and legislative framework of such a market is presented, as well as some basic organizations and regulations, that are the foundations of its proper functioning. The third chapter constitutes the literature review in which emphasis is placed on day ahead electricity spot markets (SM), as well as demand response (DR) programs. As far as the SM is concerned, there is a brief review of the available products in some of the largest European energy markets, such as those in Hungary (HUPX) and Germany (EPEX Spot). In terms of DR, a brief classification based on recent research articles is initially presented. Further emphasis is placed on the relevant techno-economic costs of DR integration, their operation, and the different degree of interest and participation of certain categories of consumers. The fourth chapter consists of the presentation of the problem, and the creation of the optimization model. The problem concerns the optimal management of an electricity providers energy portfolio, in order to fully meet the demand. This provider has three tools at its disposal: the production of energy from a natural gas power plant, the purchase of natural products by SM, and the reduction of cargo by a portion of consumers. The developed model is based on a pre-existing model, which is available to the public through the GAMS library. Specifically, two new models are created which are an evolution of the original. The fifth chapter presents, compares and analyzes, the results between the original model and the two new developed. Specifically, the second of the two new models is the final, while the first is the transition from the original. The final model developed achieves a significant economic optimization, approximately 9% in the expenses of the provider compared to the original. The last chapter constitutes the conclusions of the research and suggests the possible future research that would be worthwhile.