A heat pump water heater comprising: a refrigerant circuit including a compressor for compressing a refrigerant, a four-way valve for diverting flow of the refrigerant, a radiator , a decompressor for decompressing the refrigerant, and an evaporator ; a tank for storing water heated by the radiator ; and a controller , wherein the controller is configured to perform a heating operation for heating the water in tank wherein the refrigerant circuit is configured such that the refrigerant flows in sequence through the compressor , the four-way valve , the radiator , the decompressor , the evaporator and the four-way valve ; characterised in that the radiator is wound around a periphery of the tank , the heat pump water heater further comprises a tank temperature sensor for detecting a temperature of hot water in the tank , the controller is configured to select and to perform either a first defrost operation to melt frost on the evaporator wherein the refrigerant circuit is configured such that the refrigerant flows in sequence through the compressor , the four-way valve , the evaporator , the decompressor , the radiator and the four-way valve or a second defrost operation to melt frost on the evaporator wherein the refrigerant circuit is configured such that the refrigerant flows in sequence through the compressor , the four-way valve , the radiator , the decompressor , the evaporator and the four-way valve , the controller is configured to select and to perform the first defrost operation when the temperature of the tank is less than or equal to a predetermined value, the controller is configured to select and to perform the second defrost operation when the temperature of the tank is greater than the predetermined value. a refrigerant circuit including a compressor for compressing a refrigerant, a four-way valve for diverting flow of the refrigerant, a radiator , a decompressor for decompressing the refrigerant, and an evaporator ; a tank for storing water heated by the radiator ; and a controller , wherein the controller is configured to perform a heating operation for heating the water in tank wherein the refrigerant circuit is configured such that the refrigerant flows in sequence through the compressor , the four-way valve , the radiator , the decompressor , the evaporator and the four-way valve ; characterised in that the radiator is wound around a periphery of the tank , the heat pump water heater further comprises a tank temperature sensor for detecting a temperature of hot water in the tank , the controller is configured to select and to perform either a first defrost operation to melt frost on the evaporator wherein the refrigerant circuit is configured such that the refrigerant flows in sequence through the compressor , the four-way valve , the evaporator , the decompressor , the radiator and the four-way valve or a second defrost operation to melt frost on the evaporator wherein the refrigerant circuit is configured such that the refrigerant flows in sequence through the compressor , the four-way valve , the radiator , the decompressor , the evaporator and the four-way valve , the controller is configured to select and to perform the first defrost operation when the temperature of the tank is less than or equal to a predetermined value, the controller is configured to select and to perform the second defrost operation when the temperature of the tank is greater than the predetermined value. <EOS>