This work analyzes different modeling methodologies for balancing the electricity supply sources and the electricity demand in systems with high penetration of intermittent renewable energy sources, such as wind and run-of-river hydro. This work also explores the reasons and the circumstances where common balance approaches used by mid- and long-term energy models show significant differences in dispatched renewable sources, overestimating the renewable share in the electricity mix and underestimating the amount of CO(2) emitted by the electric system, when compared to balance methods with high time resolution. These reasons and circumstances are illustrated for the Flores island (Azores) case study which has already achieved a share of 50% of renewable energies in electricity production in 2009. (C) 2011 Elsevier Ltd. All rights reserved.