A heat pump is a device that transfers heat from a lower-temperature source to a higher-temperature sink using mechanical energy. It can provide both heating and cooling by following a cycle involving a compressor, evaporator, condenser, and expansion valve. In the winter, a heat pump extracts heat from the outdoor air and transfers it indoors by evaporating and condensing a refrigerant. In the summer, it works like a standard air conditioner, removing heat from indoor air and releasing it outside. Heat pumps are energy-efficient and measure their performance with the coefficient of performance (COP). They are most effective in moderate climates with a noticeable temperature difference. In colder climates, supplemental heating may be necessary.