In this paper, authors breakdown the specific energy consumption for seawater and brackish water desalination in an attempt to quantify the value of high permeability membranes on specific energy consumption for desalination. The trends from the analysis presented in this paper show that in both seawater and brackish desalination we are approaching thermodynamic limitations. The Gibbs free energy of mixing accounts for almost half of the specific energy consumption for seawater desalination and for brackish water desalination system losses and frictional losses account for major share of energy consumption. It is also shown that in both seawater and brackish water desalination, just high permeability membranes will not give substantial returns. Spacers, membranes, module designs and system are all areas that need to improve to make any significant enhancement in energy efficiency of reverse osmosis desalination. (C) 2015 Elsevier B.V. All rights reserved.