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.