In a real production line, rush orders arise frequently, and they must be produced more quickly than standard production runs. Rush orders may require setup changes to process the order and to resume normal production once the order is completed. Increased setup times are qualitatively understood to decrease production efficiency and to increase the energy required to produce each manufactured unit., Previous studies have not investigated the effects of rush orders on plant productivity and energy consumption. A better understanding of these effects is needed to develop production evaluation methods that can achieve continued reduction in production energy use. In this paper, we proposed a production evaluation method that theoretically formulated the relation between lot size and energy consumption rate while considering rush orders in a production line. A formulation was developed to define this relationship. The formulation was verified using simulation results. We carried out the case studies for the validity of the proposed formulation.