This study presents a simple approach to estimate the capacity of combined heat and power (CHP) units suitable for installation in high heat-to-power ratio (HPR) buildings when excess electricity cannot be sold back to the grid. The two different methods used in this approach, a graphical method and a simple equation, are capable of predicting the optimum capacity to within 7% and 15%, respectively, of the methods presented in previous studies. In particular, the simple equation has the advantage of selecting the optimum capacity using the base and peak. of the demand power load. The two easy-to-use methods can be used with any building, region, and energy-price structure that satisfies the conditions considered in this study, and will be of great use to future CHP owners. (C) 2011 Elsevier Ltd. All rights reserved.