The main purpose of this paper is to assess efficiency of the Brazilian electricity generation mix proposed in the 2020 Decennial Plan for Energy Expansion (DPEE 2020). It evaluates estimated costs, risks and CO2 emissions following the mean-variance portfolio theory. The efficiency frontier is estimated for three CO2 prices scenarios: no CO2 prices, low CO2 price and high CO2 price. The planned portfolio in Brazil presented in the DPEE 2020 is relatively close to the efficient frontier, however there is still room for risk mitigation by diversifying the energy portfolio. As there is currently no CO2 price in Brazil, the tendency is that diversification increases fossil fuel share in the energy mix, but the introduction of a CO2 price can be an option to promote renewables. This type of large general market framework can contribute to reduce market uncertainties by reducing the level of government's discretionary activism. (C) 2013 Elsevier Ltd. All rights reserved.