The fuel-use performance ratio is defined as the quantity of water produced, in kg, per 1000 kJ of the heat content of the fuel used for supplying thermal, mechanical and/or electrical energy to the desalination plant. The concept is used to evaluate the energy requirements of the major sea water conversion processes - reverse osmosis, multistage flash, multiple effect boiling (as exemplified in the vertical tube foam evaporation process), vapor compression and hybrids of the thermal processes - for three types of plants: (a) single-purpose (water only), (b) dual-purpose (power/water) involving “power loss” and (c) dual-purpose (power/water) utilizing waste heat. It is concluded that only distillation systems based on waste heat can compete with the low energy requirements of reverse osmosis.