Methodology and computer code for simulations and optimisation of the small-scale CHP plants are presented in the paper. Detailed heat and electricity demand profiles of the site are applied for analysing technical and economical effects of the plant operation and optimisation of the system. The maximum of the Net Present Value (NPV) is selected as the objective function for the optimisation procedure. Characteristics of equipment performance are taken into account to improve accuracy of the analysis. Different modes of the CHP module operation are also specified and analysed. Sample results are shown.