Short-Term Spanish Aggregated Solar Energy Forecast Nicolas Perez-Mora, Vincent Canals, Victor Martinez-Moll University of Balearic Islands, Ctra. Valldemossa Km 7.5, Palma of Majorca (07122), Spain nperez@sampol.com / v.canals@uib.es Abstract. This work presents and compare four short-term forecasting methods for hourly aggregated solar generation. The methods forecast one day ahead hourly values of Spanish solar generation. Two of the models are based on MLP network and the other two are based on NARX. The two different types of NN use to forecast the same NWP data, comprising solar irradiation and the cloudiness index weighted with the installed solar power for the whole country. In addition of the NWP data the models are fed with the aggregated solar ener- gy generation in hourly step given by the System Operator. The results of the two types of NN are compared and discussed in the conclu- sions as much as the error variability along the day hours. The results obtained by the four methods are evaluated, concluding that the most accurate result is the one given by the developed NARX irradiance forecast method; achieving the lowest one day-ahead Mean Average Daily Error of 16.64%. Keywords: Forecasting, Solar Radiation, Solar Forecasting, Energy Market, Neural Networks, Time Series 1 Introduction The installation of power plants based on renewable energy sources have increased exponentially in the recent years [1]. The fuel-based energy generation price increase along with the subsidies promoting the construction of clean energies made the instal- lation of renewables a profitable business for many particulars and companies [2]. The expansion of these technologies and the policies adopted by the Spanish national gov- ernment to integrate them into the electrical power system has brought a fussy market situation. The integration of the electric energy generated by these power plants into electric power system is priority. According with the current legislation, the power generated by green sources is fed into the system in preferential order within the Spanish energy generation mix. This energy mix is result of a counter clockwise energy auction held one day-ahead.