The purpose of this study is to find plant utilities capacity for economical operation of combined heat & power by reducing energy expenditure. Using a numerical simulation program CHPSIM, a comparative analysis of additional heat expenditure (AHE) of combined heat & power plant in relation to size of district heating has been performed within the comparison of the difference capacity of gas turbine and steam turbine . As a results, if a 105.2MW gas turbine (exhaust gas temp ; 540$^{circ}C$) installed in CHP plant can reduced 17-18% yearly the AHE than 75MW gas turbine (520$^{circ}C$) installed. If a 130-150MW gas turbine (560-580$^{circ}C$) installed, can reduced 34.7-35.8% of the yearly AHE.