Air source heat pump is an electrical device that extracts heat from one place and transfers it to another. The heat pump is not a new technology; It has been used in China and around the world for decades. Refrigerators and air conditioners are both common examples of this technology.
Air source heat pumps are a well established technology for water heating. The same principal of transferring heat is at work in heat pump water heaters except that they extract heat from air (indoor, exhaust or outdoor air) and deliver it to water. Some models come as a complete package including tank and back-up resistance water heating elements while others work as an adjunct to a conventional water heater.
Because it extracts heat from air, the heat pump water heaters delivers about triple to quadruple the heat for the same electricity cost as a conventional electric resistance water heater. To produce the hot water of the same amount and temperature, the amount of energy consumption can be 1/3 of the amount of energy consumed by the gas water heater and 2/3 that of regular solar energy water heater, so heat pump water heaters is the most energy-saving water heater.