Unlike conventional electric storage water heaters which use electricity to heat water directly, heat pump storage water heaters use electricity only to operate a pump that circulates refrigerants around the system. This refrigerant collects heat from the surrounding atmosphere and transfers it to the water. In effect, the heat pump works like a refrigerator in reverse. Heat pump water heaters are effective because they use a refrigerant with a very low boiling point to allow the transfer of thermal energy between the surroundings and the stored water. As long as the temperature of the surrounding atmosphere is higher than the cold refrigerant, the heat pump will be able to absorb heat and transfer it to the water.