Dedicated heat pump water heaters (HPWPs) are being marketed. Product literature and field studies imply that the COP of the HPWHs is mainly affected by ambient air temperatures and only slightly affected by the temperature of the water supply. Based on the experimental results presented, the operational efficiency of HPWHs is also affected by quantity of hot water drawoff and capacity of storage tank. These two factors influence water temperature entering the HPWH. The suggestion is made that standard tests for predicting HPWH operational performance should not be confined to just cold-start recovery tests.