While most of Colombia's power comes from large-scale hydroelectricity generation, it still depends on fossil-fuel-based technologies. Alternative cleaner energy sources have been largely neglected despite their abundance and the likely complementarities between different renewable resources. This limited mix of energy sources has made the system vulnerable to unpredictable and recurrent drought conditions (El Nino) occurring every 4-5 years. In the past, El Nifio brought high energy costs and power supply losses. This paper proposes an optimisation model to study the insertion of renewable energy systems (RES) into the Colombian electricity sector. The model considers a cost-based generation competition between conventional technologies (hydro and thermal) and alternative RES (solar photovoltaic (PV) and wind). It aims at minimising system costs, CO2 emissions, and the number of blackout events. The model is solved by following two procedures known as Implicit Stochastic Optimisation (ISO) and Robust Optimisation (RO), and the results indicate that alternative renewables should replace all fossil-fuel-based technologies for economic and environmental reasons. Solar PV seems particularly promising to expand system capacity, as it contributes to both the reduction of the overall system costs and to the ability of the system to cope with extreme dry weather conditions. (C) 2018 Elsevier Ltd. All rights reserved.