Energy markets seek the match of supply and demand through competitive mechanisms, balancing generators offers and forecast demand for the day ahead or for the medium and long term. Due to the stochasticity involved, it is necessary to develop optimization under uncertainty models that guarantee the success of the planning and operation of the system. Regulators are responsible for designing effective rules and monitoring the market behavior, in order to update them whenever necessary. In Brazil, the modernization agenda of the MME – Ministry of Mines and Energy, scheduled for the next decade, has as one of its main challenges the update of the price formation mechanism, which could be by offers, instead of the current model by audited costs. On the one hand, agents offers may capture their own perceptions of risk and should encourage competition; on the other hand, they enable the exercise of market power by competitors, which must be mitigated by the market monitoring body. To this end, it is necessary to develop a customized design for the Brazilian reality, due to the peculiarities of our hydrothermal electrical matrix and other specificities, such as the cascades of hydroelectric plants with multiple owners and the increasing penetration of intermittent renewable resources. In this sense, the form of aggregation of energy prices plays a relevant role in mitigating the exercise of market power, and its relationship with the other elements of the adopted market design are crucial to ensure competition. Another essential point to guarantee the success of the competition is, as mentioned before, the monitoring of market power by the competent authorities. One of the ex-ante tools widely used for this purpose are equilibrium models, particularly the Nash Equilibrium, whose concepts are based on the Theory of Non-Cooperative Games. In this approach, the behavior of competitors is simulated, based on their economic rationality, in search of na equilibrium point in which none of them is interested in changing their offers, as it is not possible to increase their revenue. Through the modeling and simulation of the market under study, it is possible to calculate such equilibrium point and compare situations in which competitors have the power to change their offers to maximize their revenues with situations in which they are induced to present offers consistent with their marginal energy production costs, indicating possibilities of unilateral exercise of market power. The present work aims to investigate the impact of price aggregation in a zonal market with nodal dispatch on the exercise of market power by competitors, making use of equilibrium models and applying them to a Brazilian market with price formed by offers. To achieve this goal, a Stackelberg Equilibrium will be implemented in a bilevel problem, where the upper problem aims at maximizing the revenue of the leading competitor and the lower one deals with the energy dispatch as the reaction of the other players. The Nash Equilibrium will be reached by implementing iteratively the Stackelberg Equilibrium for all players and, in this sense, the Nikaido-Isoda function will be taken into account while updating the offers. The bilinear function presented in the Mathematical Program with Equilibrium Constraints – MPEC described will be linearized by the Fortuny-Amat-McCarl approach, with the replacement of the second level by its Karush-Kuhn-Tucker – KKT conditions. Finally, the model will be applied to a system with data extracted from the brazilian network, allowing the study of the impact of zonal and nodal price formation on the exercise of market power.