Investing more in renewable energy sources and using this in a rational and efficient way is vital for the sustainable growth of world. Energy efficiency (EE) will play an increasingly important role in future generations. The aim of this work is to estimate how much the PNEf (National Plan for Energy Efficiency) launched by the Brazilian government in 2011 will save over the next 5 years by avoiding the construction of additional power plants, as well as the amount of the CO2 emission. The marginal operating cost is computed for medium term planning of the dispatching of power plants in the hydro-thermal system using Stochastic Dynamic Dual Programming, after incorporating stochastic energy efficiencies into the demand for electricity. We demonstrate that even for a modest improvement in energy efficiency (< 1% per year), the savings over the next 5 years range from R$ 237 million in the conservative scenario to R$ 268 million(1) in the optimistic scenario. By comparison the new Belo Monte hydro-electric plant will cost R$ 26 billion to be repaid over a 30 year period (i.e. R$ 867 million in 5 years). So in Brazil EE policies are preferable to building a new power plant. (C) 2013 Elsevier Ltd. All rights reserved.