Air source heat pump (ASHP) units are widely used in recent years, and reverse cycle defrosting becomes the most popular method to solve their undesired frosting problem. During defrosting, a transient and nonlinear heat and mass transfer procession, the metal energy stored in the indoor and outdoor coils are varying as their temperature fluctuations. On the other hand, authors have previously confirmed the effects of melted frost and metal energy storage on system defrosting performance. However, detailed energy transfer procession without melted frost influence is still not identified. This fundamental problem directly affects the development and modification of two coils in a novel ASHP unit or an existing one. Consequently, basing on frost evenly accumulated on each circuit's surface, two cases were thereby designed in this study. Experimental results show that, the heating supply of indoor air thermal energy contributed about 80% of the total energy usage for defrosting, nearly 90% of energy consumed on frost melting and ambient air heating, respectively. After the total area of outdoor coil was enlarged by 50%, the metal energy storage effect was changed from -0.44% to -3.67%. Meanwhile, defrosting efficiency was improved by 11.66%, from 47.13% to 58.79%. Contributions of this study can effectively guide the design optimization of an ASHP unit, improve occupant's thermal comfort and promote the energy saving in buildings and industry. (C) 2018 Elsevier B.V. All rights reserved.