Decision making under uncertainty is a difficult task in most areas. Investment decisions for combined heat and power production (CHP) are certainly one of the areas where it is difficult to find an optimal solution since the payback period is several years and parameters change due to different perturbing factors of economic and mostly political nature. CHP is one of the most effective measures for saving primary energy and reduction of greenhouse gas emissions. The implementation of EU directives on the promotion of cogeneration based on useful heat demand in the internal energy market will accelerate CHP installation. The expected number of small CHP installations will be very high in the near future. A quick, reliable and simple toot for economic evaluation of small CHP systems is required. Since evaluation is normally made by sophisticated economic computer models which are rather expensive, a simple point estimate economic model was developed which was later upgraded by risk methodology to give more informative results for better decision making. This paper presents a reliable computer model entitled "Computer program for economic evaluation analysis of CHP" as a tool for analysis and economic evaluation of small CHP systems with the aim of helping the decision maker. The paper describes two methods for calculation of the sensitivity of the economic results to changes of input parameters and the uncertainty of the results: the classic/static method and the risk method. The computer program uses risk methodology by applying @RISK software on an existing conventional economic model. The use of risk methodology for economic evaluation can improve decisions by incorporating all possible information (knowledge), which cannot be done in the conventional economic model due to its limitations. The methodology was tested on the case of a CHP used in a smaller hospital. (c) 2007 Elsevier Ltd. All rights reserved.