Both energy and water are important resources for human and should be investigated in an integrated way. The paper examines the implications of improving energy efficiency for water resources to contribute to the understanding of energy-water nexus. The paper first develops a mathematical model to link energy efficiency and water withdrawals and consumption. Using the model and the UK energy and water data, the paper then investigates the implications of improving energy efficiency of thermal power technology for freshwater, tidal and sea water in the UK. Results show that when the energy efficiency increases averagely by 10%, the total water withdrawals decrease by 1.65 x 10(6) Megaliter (ML) per year, and the total water consumption reduces by 2.89 x 10(4) ML per year. The saved freshwater withdrawals account for 0.43% of total public water supply in England and Wales in 2013, and could support 138,462 households. The water resources have different sensitivity to the improvement of energy efficiency. The sea water withdrawals are the most sensitive to the improvement of energy efficiency, followed by tidal sea water withdrawals and freshwater withdrawals. Results provide significant informative implications for energy-water nexus, and suggests that it is time to consider improving energy efficiency to address the emerging energy and water challenges. (C) 2017 Elsevier Ltd. All rights reserved.