In a combined heat and power (CHP) plant, the heating and electrical loads are mutually exclusive and cannot be chosen independent of each other. This paper reports on the mapping of the ranges of the heat-to-power ratios (HPRs) for given fractions of the electrical output. Alternatives for matching the HPRs of the CHP plant (source) and target process (load) are also discussed, It is shown that the maximum overall efficiency of CHP plants is about 90%. Design equations for the overall efficiency and HPRs of Diesel generating sets, boiler-steam turbine-generator units and combined cycle plants have been provided as a function of the fraction of the electrical output for optimal and rational design of the systems,