Air source heat pump units have found their wide applications in recent decades due to their high efficiency and low environmental pollution. To solve their undesired frosting problem, reverse cycle defrosting is always employed. As a transient and nonlinear heat and mass transfer procession, defrosting performance directly affects the occupants' thermal comfort. During defrosting, the metal energy storage values of indoor and outdoor coils are varied as their temperature fluctuations. It is therefore necessary to investigate the energy transfer procession in an air source heat pump unit and the effect of metal energy storage during defrosting. However, scarce of attentions were paid to this fundamental problem. In this study, two experimental cases with two-working-circuit and three-working-circuit outdoor coils were conducted basing on frost evenly accumulated on their surfaces. After four types of energy supply and five types of energy consumption during defrosting were calculated,. a qualitative and quantitative evaluation on the metal energy storage effect was then given. As concluded, after the outdoor coil enlarged 50%, the metal energy storage effect can be changed from positive (0.33%) to negative (-2.18%). The percentages of energy consumed on melting frost and vaporizing retained water were both increased. Defrosting efficiency was improved about 6.08%, from 42.26% to 48.34%. Contributions of this study can effectively guide the design optimization of indoor and outdoor coils and promote the energy saving for air source heat pump units. (C) 2017 Elsevier Ltd. All rights reserved.