Many Public Administrations are supporting the installation of Renewable Energy Systems (RE, namely wind and photovoltaic), which provide almost carbon free sources of electricity, aiming to curtail CO2 emissions of the power generation sector. However, the real effect in terms of carbon dioxide reductions is still unclear, since the uncertainty and variability characterizing RE must be balanced by conventional generators. The model presented here simulates the technical constraints Of power plants and the economic framework to be found in a national electricity market, and estimates carbon dioxide missions. The results show that the reduction of CO2 emissions is lower than expected considering the amount of energy produced from renewable sources, and is related to the level of RE penetration and the season of the year; in summer the reduction is slightly greater, because of the higher production by Combined Cycle Gas Turbines (CCGTs) and the consequent decrease of that generated by the more pollutant coal power plants. The amount of reserve allocated by the Transmission System Operator (TSO) and the cycling on hourly basis have negligible effects on the carbon intensity of electricity generation. In the Italian system 1 kWh from RE displaces approximately 0.8 kWh from conventional power plants. (C) 2016 Elsevier Ltd. All rights reserved.