Combined heat and power (CHP) is a well-known technique for producing heat and power simultaneously onsite. However, the micro level of this technology has just been recently introduced around the world, and expected to widely spread. Therefore, identifying the optimal size of such systems would give them the potential for being more beneficial. In this paper, a generic deterministic linear programming model, which aims to minimize expected annual cost of the system, has been developed. This model is capable of optimally determining the optimal size (electrical rating) of a micro CHP (mu CHP) unit and the optimal size (thermal rating) of a back-up heater for any given residential demand regardless of the type of mu CHP technology. An investigation has been conducted to identify economically the optimal mu CHP investment for three typical residential dwellings in England. The four candidate mu CHP technologies that have been considered in this paper are: internal combustion engine (ICE), Stirling engine (SE), solid oxide fuel cell (SOFC), and proton exchange membrane fuel cell (PEMFC). Sensitivity analyses have been conducted to understand the influence of some important key parameters on decision making regarding the deployment of residential mu CHP systems. (C) 2011 Elsevier B.V. All rights reserved.