ORC is generally utilized to generate electricity from heat sources with a relatively low temperature difference. Typical low-grade heat sources utilized as sensible heat are the discharged heat from industrial plants, geothermal, hot-spring and ocean thermal gradients. In the design of the heat balance for the energy conversion systems, the maximization of available work per the heat source quantity is extremely important. The condition for maximized work output is determined by the balance of the donated heat duty from the heat source and the thermal efficiency of the heat engine. Since the thermal efficiency mainly depends on the available temperature difference of the heat source, the thermal efficiency itself does not affect the performance of the energy conversion system. Alternative performance evaluation methods used for energy efficiency are generally complicated for industrial and public use, although eventually thermal efficiency is still used in the evaluation of systems in spite of its inherent irrelevance. Therefore, an alternative definition of thermal efficiency for the performance evaluation of systems is inevitably required. Firstly, this research clarifies the potential energy of the heat sources using the dead state as the thermal equilibrium state of the finite heat sources instead of the infinite quantity of the environmental temperature. Secondly, it proposes that the normalized thermal efficiency of energy conversion be defined as the ratio of the work over the potential energy. The normalized thermal efficiency coincides with a change in the work from the heat engine. The result shows that the introduction of normalized thermal efficiency is significantly effective for the evaluation of energy conversion systems based on two different temperature streams. At the stage of the design of the heat and mass balance in the ORC system, optimization by the maximizing the normalized thermal efficiency will yield the effective power output in limited heat sources.