Direct hot water production consumes about 4% of the total energy use in Hong Kong, and about 20% when considering only the domestic sector. For water heating the energy sources are mostly town gas, liquefied petroleum gas and electricity. The use of heat pump or solar water heating, particularly the solar-assisted heat pump options, is not popular. In this paper, the potential application of a unitary type direct-expansion solar-assisted heat pump (DX-SAHP) system was examined. A numerical model of the DX-SAHP system was first introduced. From the simulation results with the use of the Typical Meteorological Year (TMY) weather data of Hong Kong, the system was found achieving a year-average coefficient of performance (COP) of 6.46, which is much better than the conventional heat pump system performance. The potential use of DX-SAHP therefore deserves further evaluation. (C) 2009 Elsevier Ltd. All rights reserved.