The long-term planning of a future electricity supply system requires data about future demand. Planners who use the conventional planning method forecast future demand by observing past trends or alternatively by developing scenarios and then selecting the scenario considered to be the most likely to occur. This method, however, fails to include future uncertainties. To consider such uncertainties, the scenario planning method may be used. This study uses this method to devise a long-term electricity supply plan for the Java-Madura Bali electricity system. It was found that the scenario planning method could save up to US$3.5 billion over a 15-year period if the method was applied right at the beginning of the period. In the case of the Java-Madura-Bali system, which currently has excess installed capacity, the scenario planning method does not provide such large benefits. It was also found that introducing integrated coal gasification combined cycle and advanced gas combined cycle units would reduce greenhouse gas emissions from the Java-Madura-Bali system by approximately 230 million tonnes or 15% compared to a business-as-usual (BAU) scenario over a 15-year planning timeframe. The abatement cost was found to be US$4 per tonne Of CO2. Crown Copyright (c) 2006 Published by Elsevier Ltd. All rights reserved.