Optimizing power generation from waste heat in large industrial facilities such as petroleum refineries by utilizing a subset of all available hot source streams selected based, in part, on considerations for example, capital cost, ease of operation, economics of scale power generation, a number of ORC machines to be operated, operating conditions of each ORC machine, combinations of them, or other considerations are described. Recognizing that several subsets of hot sources can be identified from among the available hot sources in a large petroleum refinery, subsets of hot sources that are optimized to provide waste heat to one or more ORC machines for power generation are also described. Further, recognizing that the utilization of waste heat from all available hot sources in a mega-sitesuch as a petroleum refinery and aromatics complex is not necessarily or not always the best option, hot source units in petroleum refineries from which waste heat can be consolidated to power the one or more ORC machines are identified.