**Abstract**

A major fraction of secondary energy consumed for our daily activities, such as electricity and low-grade heat sources, emanates from the conversion of fossil fuels in power plants. In the seawater desalination processes, the energy efficiency is usually expressed in kWh electricity or kWh of low-grade heat per unit volume of water produced. Although kWh energy unit provides a quantitative measure of input energy, it has subtly omitted the embedded quality of supplied energy to desalination plants. In assuming the equivalency across dissimilar energy forms, it results in a thermodynamic misconception that has eluded the desalination industry hitherto, i.e., not all units of derived energy are created equal. An incomplete energy efficacy approach may result in the inferior selection of desalination processes to be deployed; —a phenomenon observed in the trend of installed desalination capacity globally. Operating a less efficient desalination plant over its lifespan would create much economic burdens including a higher unit cost of water, higher CO2 emissions and greater brine discharge to the environment. This book chapter clarifies the key concept and a thermodynamic framework to rectify the misconception in energy consumption, permitting energy planners and designers to optimize deployment of future desalination plants for energy sustainability. We have derived conversion factors to convert assorted derived energies into standard primary energy for fair comparison.

**Keywords:** sustainable desalination, thermodynamic limit, universal performance ratio, primary energy
