The operating power reserve is the share of generation responsible for ensuring the reliability of electrical systems in the event of equipment contingencies, as well as errors in load and generation forecasting. The increasing penetration of variable renewable sources in electrical grids, such as wind and solar photovoltaic sources, has been adding even more uncertainty to planning, operation scheduling and the operation itself. Therefore, the method for calculating the operating power reserve should follow the transformations of the system, requiring the representation of uncertainties and consideration of equipment failure rates for adequate sizing. Oversizing the operating reserve incurs high costs to the consumer, while undersizing exposes the system to risks. Given the stochastic characteristics of the system, the use of probabilistic methods is more appropriate for sizing the operating reserve. In this dissertation, the main objective is to apply Monte Carlo simulation to estimate reliability indices for the Brazilian electrical system. Deviations from the wind load and generation forecast are represented by distributions obtained from historical data. The power generations of the adopted hydroelectric and thermoelectric plants are based on the results of the energy optimization model, used in the daily programming of the Brazilian electrical system operation. Several tests are carried out in order to evaluate the impacts on the reliability indices of the Brazilian electrical system.