The increasing levels of Variable Renewable Energies (VRE) require additional levels of flexibility in present power systems due to their intermittency and the resulting needs of reserve for increasing/reducing the generation with enough speed to maintain the system balance. In this context, the traditional long-term hydrothermal optimal dispatch models are not sufficient to adequately represent this situation, particularly in systems with hydro reservoirs that can store water from one period to another. These models do not use sequential representation of time periods (on the contrary, they work with load blocks) and they do not incorporate most of the non-linear constraints to model specific characteristics such as: minimum power, ramp up/down rates, stabilization times, minimum time in service, minimum down-time, etc. Therefore, more detailed models must be used to comprehensively represent the impact of VRE in the system operation. To assess the true cost of power system operation with high adoption of VRE, this work develops a detailed multi-bus hourly unit commitment model with all non-linear constraints, showing the importance of this type of model for assessing cycling and start-up in the new VRE paradigm.