A new concept of ideal RO process is introduced in this study for a more appropriate assessment of energy efficiency of water desalination, in which all the extra energy above the thermodynamic minimum is spent to maintain the required permeate flux. A pressure-recovery diagram was developed as a graphical method for better analyses and presentations of energy consumption in cross flow RO. It was demonstrated that the total energy input to a cross flow RO was much higher than the thermodynamic minimal energy for water desalination. Aside from the energy that remained in the retentate stream, a substantial amount of additional energy was needed in cross flow RO to maintain an economically meaningful nonzero permeate flux and overcome the elevated osmotic pressure due to salt accumulation along the membrane channel (configuration associated energy). The configuration associated energy became dominant at high recoveries and set the ultimate limit on the energy requirement of cross flow RO, which could not be reduced by further improvement in membrane permeability. Finally, the energy efficiency of cross flow RO was compared to an ideal RO process in which the configuration energy was completely eliminated.