The deployment of renewable energy is considered as one of the important alternative solutions for reducing greenhouse gas emissions caused by electricity generation. As one of the key measures for promoting the deployment of renewable energy, the Renewable Portfolio Standard (RPS), which obligates power operators with a power equipment capacity of 500 MW or larger (excluding renewable energy equipment) to supply over a specific portion of their total electricity generation output in the form of renewable energy, was enacted in South Korea in 2012. However, there are still many disputes concerning the appropriate renewable energy deployment goal and the optimum portfolio for each renewable energy source which enable to minimize the economic burden while achieving the deployment target. Therefore, this paper intends to deduce the optimum portfolio up to 2050 based on explicit representation of renewable energy technologies and analyze the cost effect using the bottom–up energy system analysis model of electricity generation sector in South Korea. The result indicates that the potential for a reduction of the cost of PV was the highest and that, in the long term, it would account for the highest portion of the renewable energy portfolio as it was the most cost competitive technology. The additional cost for increasing the renewable deployment target 20% compared with the 3rd Energy Basic Plan was predicted to be between 107 and 115 USD/MW h. Therefore the renewable energy will be able to play a greater role in reducing GHG emission if the R&D to reduce investment and operation cost of PV and wind power is conducted successfully.