Heat pumps are relatively simple appliances. They share similar components to refrigerators and air conditioners and effectively move an external source of heat to where it is needed, such as in buildings for heating or the production of hot water. Their key value is efficiency; for each unit of electricity consumed to operate them, they produce multiple units of usable heat. Because of this, they require much less energy input for a similar heating outcome compared to combustion technologies, making them naturally cleaner and generally cost effective to run.