This paper presents a simulation software initially developed by the author for educational purposes. Thecomputational tool supports the design of power systems with large penetration by renewable energysources. In particular, the problematic of power intermittency and its counter strategies are targeted. Themain innovation of this simulation is the detailed transient analysis of the essential balance betweenpower generation and consumption. Even so, the focus of the simulation tool is simple usage andinterpretation of results, it successfully captures important characteristics of renewable power systems.The user selects the composition of a power system from conventional power plants, photovoltaic,windpower and tidal power. Following system definition, power generation and power demand arecalculated based on local weather data. Energy storage can be added to balance mismatches betweenpower demand and supply. Following the completion of a simulation system autonomy, carbon emissionand electricity cost are evaluated to assess the performance of energy systems.