We report a LDC (load duration curve) method to determine the optimum size of CHP (combined heat and power) units. The method gives the appropriate capacity graphically from the LDC of a building's heating demand. The method can be applied to the most common CHP units that are connected to the electrical grid, installed with thermal storage and auxiliary heat sources, and operated by a traditional heat-led strategy. The LDC method is simple and requires less information than existing sizing methods. Our method is in agreement with existing methods within 2.7-12.6% for internal combustion engine-driven CHP units, and 17.1-32.1% for Stirling engine-driven CHP units. (C) 2013 Elsevier Ltd. All rights reserved.