An analytical study is elaborated to determine the principal factors that affect the optimum size of combined heat and power (CHP) units in residential applications. The optimum thermal capacity of CHP was found to correspond to 30-50% of the maximum heating load, but proved to be stronger correlated to the annual heating degree-days instead of the minimum temperature of the area. The optimum capacity was found dependent on the balance temperature of the dwelling; the shape of the ambient temperature duration curve; the economy of scale exponent of CHP units cost; the variation of CHP part load efficiencies. On the contrary, prevailing energy prices proved to have a minor effect on the optimum CHP capacity. (C) 2015 Elsevier Ltd. All rights reserved.