The increasing amount of Renewable Energy Sources (RES) injected into Power Systems results in an increasing uncertainty in electricity generation, which has to be taken into account in energy market simulation models. The stochastic medium term (time horizon from one week to one whole year) market simulator s-MTSIM has been developed in a collaboration between RSE S.p.A. and University of Bergamo, as a stochastic extension of the existing deterministic market simulator MTSIM, originally developed by RSE. s-MTSIM solves the hourly large scale stochastic Unit Commitment (UC) problem for a Zonal Power System by means of Stochastic Linear Programming (LP) techniques and a heuristic procedure to cope with the Mixed Integer intrinsic nature of UC problem. RES generation uncertainty is taken into account by building different RES generation scenarios. Each scenario has a corresponding probability and it is derived from a statistical characterization of RES generation profiles performed on the historical data available on the Italian Transmission System Operator web site, with zonal and hourly detail. This work compares two different UC’s for thermal generation units in a power system with high levels of RES penetration in a time horizon of one week characterized by low demand levels, in order to highlight the stress caused on the whole system by RES uncertainty. The first UC is calculated stochastically by means of s-MTSIM market simulator; the second one is, instead, calculated deterministically considering only one possible RES generation scenario. Then, the two UC’s just calculated are tested on the same “actual RES generation” profile so that it is possible to compare their performances with respect to the costs of generation, CO2 emissions, fuel consumption, congestion of inter-zonal connections and zonal prices.