Operational flexibility appears to be a significant paradigm shift for conventional thermal power plants when power demand keeps fluctuating and due to the integration of renewable energy sources with the existing power systems. This study considers heat rate variation with load factor in the design and targeting of an optimal off-grid power plant with generation flexibility. The effect of heat rate in power system is investigated in this study via the comparison between two case scenarios: (1) plant with constant heat rate regardless of plant load factor and (2) plant with varying heat rate depending on load factor. The models formulated for each case scenario are solved mathematically using optimisation software, General Algebraic Modelling System (GAMS). The models will decide which load factor to be operated at each hour (the flexible generation profile) and the corresponding heat rate value that will result in optimal plant configuration and operation. The result evaluating the plant performance in term of energy and operational efficiencies are slightly different between both case scenarios.