This paper presents the numerical simulation of a solar assisted ground coupled heat pump (SAGCHP) system which can provide both space heating and domestic hot water (DHW). The optimization process is completed on the TRNSYS based platform by simulating the influence of solar collector area on the total borehole length and system performance. Simulations are also carried out under different meteorological conditions to explore the applicability of the proposed SAGCHP system in northern China. The simulating results show that the optimized system under the specified load conditions has a collector area of 40 m(2) and a borehole length of 264 m. The annual total heat extraction plus 75% of the hot water requirement can be provided by solar energy in the optimized design. Furthermore, the energy balance of the optimized design is confirmed with a minor difference of 0.75%, and the system is proved more efficient and economical for its application in Beijing area. (C) 2011 Elsevier Ltd. All rights reserved.