In the respiration equation, the energy act as a "product" because the energy is generated from the oxidation of the stored molecule which is glucose.
Explanation:
During photosynthesis the energy act as a reactant to generate sugar molecules but during respiration the produced sugar molecule is used to produce energy through oxidative phosphorylation. Hence, during the respiration energy acts as a product rather than a reactant because the product of respiration is ATP molecule which is a high energy product of respiration.