Air　source　heat　pump　(ASHP)　units　have　been　widely　used　for　space　heating　in　recent　years.　While　a　space　heating　ASHP　unit　is　normally　rated　at　its　nominal　operating　condition,　however,　during　its　actual　space　heating　operation,　it　rarely　works　at　the　nominal　condition.　The　actual　output　heating　capacity　can　remarkably　deviate　from　that　at　the　nominal　condition,　due　to　the　influences　of　ambient　air　temperature　and　frosting-defrosting　operation.　Therefore,　to　enable　a　comprehensive　and　convenient　evaluation　of　the　operating　performance　of　ASHP　units　with　frosting-defrosting　operation　and　to　provide　designers　with　appropriate　design　guidelines　to　size　a　space　heating　ASHP　unit,　a　new　performance　index　has　been　proposed.　The　new　performance　index,　epsilon(NL)　or　the　loss　coefficient　in　the　nominal　output　heating　energy,　was　actually　based　on　the　nominal　output　heating　capacity　of　ASHP　units,　which　was　readily　available　and　stayed　unchanged　irrespective　when　and　where　ASHP　units　were　operated.　In　this　paper,　the　definition　of　epsilon(NL)　was　firstly　given,　following　a　detailed　explanation　of　the　frosting-defrosting　operation　of　an　ASHP　unit.　Secondly,　a　GRNN　model　for　predicting　the　CND　of　a　field　ASHP　unit　was　established　following　a　correlation　analysis　using　a　total　of　epsilon(NL)　groups　of　field　measured　data　from　the　field　ASHP　unit.　Finally,　a　modeling　study　using　the　developed　GRNN　model　was　carried　out,　and　the　study　results　suggested　that　when　an　ASHP　unit　was　operated　at　a　fixed　ambient　condition,　defrosting　initiating　time　would　affect　the　epsilon(NL)　and　there　may　exist　an　optimal　defrosting　initiating　time　at　which　epsilon(NL)　was　at　its　minimum.　The　model　results　also　demonstrated　that　an　increase　in　ambient　air　relative　humidity　or　a　decrease　in　ambient　air　temperature　would　result　in　an　increase　in　epsilon(NL),　when　an　ASHP　unit　was　operated　with　a　fixed　defrosting　initiating　time.　(C)　2018　Elsevier　B.V.　All　rights　reserved.