Several electricity systems supply significant proportions of electricity from weather dependent renewable sources. Different quantification methods have estimated the associated historical savings of fuel and CO2 emissions. Primary energy equivalent and econometric methods do not readily quantify factors like operational changes to fossil fuel generation arising from the integration renewable energy. Dispatch models can overcome these limitations, but are generally applied to future scenarios. A dispatch model is applied to ex-post data for the 2012 All Island system in Ireland. Renewable electricity accounted for 20.4% of total generation, 15.8% from wind. The results show renewable generation averted a 26% increase in fossil fuels (valued at (sic)297 million) and avoided an 18% increase in CO2 emissions (2.85 MtCO(2)), as compared to the simulated 2012 system without renewable generation. Wind averted 20% increase in fossil fuel generation and a 14% increase in CO2 emissions (2.33 MtCO(2)). Each MWh of renewable electricity avoided on average 0.43 tCO(2) with wind avoiding 0.46 tCO(2)/MWh. Additional renewable related balancing requirements had minor impacts on fossil fuel generation efficiency; CO2 production rates increased by < 2%. Policy measures to alleviate network congestion, increase system flexibility and increase financial penalties on emissions can increase savings from renewable generation. (C) 2015 Elsevier Ltd. All rights reserved.