Domestic CHP (combined heat and power) generation is one new application of the ORC (organic Rankine cycle). An environment temperature fluctuation of 40 degrees C through the year is common in many areas, where the consumer's demand on heat follows a seasonal cycle. In no demand periods the ORC shall work under lower condensation temperature for more efficient power generation. Off-design operation will be executed, accompanied with a degraded performance of the ORC components especially the expander. The design of the condensation temperature herein becomes crucial. It influences the ORC efficiency in both the CHP and SPG (solo power generation) modes. If the condensation temperature is designed simply based on the CHP mode, the power conversion in the SPG mode will suffer from low expander efficiency. An optimum design of the condensation temperature involves a compromise between the power outputs in the two modes. This paper aims to determine the optimum design condensation temperature for the ORC-CHP system. A new concept, namely the threshold condensation temperature, is introduced and found to be important to the design and operation strategies of the system. The results indicate that via a careful design of the condensation temperature, the annual power output can be increased by 50%. (C) 2014 Elsevier Ltd. All rights reserved.