Large-scale power plants and energy producers discard massive heat energy that goes unused. Energy efficiency techniques such as insulation or properly maintained heating systems are less likely when it requires high investments. While "conservation" generally refers to the reduction of total energy usage, "efficiency" means increasing the yield per unit of energy. In many cases, the energy needed to heat water or refrigerate products can also be reduced through conservation and application of more efficient technologies. Heat pump water heaters use electricity to "move" heat from one place to another instead of generating heat directly. Therefore, they can be two to three times more energy-efficient than conventional electric resistance water heaters. This book examines the full application of energy-efficient water-heating systems, focused on heat pump devices. The main objective is to present a brief introduction of the working principles of this equipment, and concentrate on descriptions and applications where both "purposes" of the system are used, the heating and the cooling components, maximising its effectiveness.