The efficiency capacity of the generating unit equipment has an efficiency value corresponding to the product design. As time goes by the value of the capacity of the generating unit equipment will be relatively decreased due to the operation and maintenance period, thus causing the efficiency of the generating unit to decrease. One of the causes of decreased efficiency in generating units occurs in the HRSG system. At PT. Indonesia Power UJP PLTGU Cilegon has 2 HRSG under the same brand and type, but the efficiency value obtained from both HRSGs has an efficiency difference of about 3% in March 2018Judging from the annual efficiency data, by 2015 the HRSG 1 efficiency is 87.35% and the HRSG 2 efficiency is 85.59%, in 2016 the efficiency of HRSG 1 is 86.76% and the efficiency of HRSG 2 is 85.24% in 2017 the efficiency of HRSG 1 was 87.54% and the efficiency of HRSG 2 was 85.12%, while in 2018 the efficiency of HRSG 1 was 87.21 and the efficiency in HRSG 2 was 84.69%. Causes of impairment of efficiency on HRSG 1 and HRSG 2 include leakage of HRSG tube, absorption of heat not optimal on one HRSG, heat loses between exhaust gas turbine with HRSG inlet. The purpose of this study analyzed the absorption of heat that is not optimal on HRSG. The method used in this study is to compare the data between HRSG efficiency in March 2018 and HRSG efficiency data in previous years, which aims to find out how much the efficiency decrease that occurred. From the calculation results obtained the value of incoming heat is almost the same that is 1370221763,17 kJ / h for HRSG 1 while 1370226150,97 for HRSG 2, but the value of heat absorbed by HRSG 1 of 1198645622019 kJ / h while the heat is absorbed by HRSG2 of 1151184244888 kJ / h. the largest heat absorption difference occurred in the HP system with the percentage of heat absorption 59,272% for HRSG 1 and 55,085% for HRSG 2. In this study it can be concluded that the difference in efficiency on both HRSG caused by different values of incoming heat and heat absorbed by the HRSG system own.