Abstract Estimation of cogeneration potential prior to the design of the total site utility system is vital to set targets on site fuel demand and steam flowrate as well as heat and power production. This paper presents in detail the Iterative Bottom-to-Top Model (IBTM) as a new shaftwork targeting model which facilitates the targeting stage. The IBTM calculates the temperature of steam mains, steam flowrate and shaft power generated by the steam turbines in expansion zones of the site utility grand composite curve from bottom to top using a simple steam turbine expansion model with a constant isentropic efficiency. Unlike the existing models, IBTM provides the degree of superheat at process steam generators and steam boiler house. Through a case study of a refinery plant, the applicability of the IBTM in total site analysis is presented. It has been shown that the features of IBTM make it preferable for its implementation in flexible targeting tools to set realistic targets on the site fuel demand and the cogeneration at the early stages of design.