A heat pump is a device used to warm and sometimes also cool buildings by transferring thermal energy from a cooler space to a warmer space using the refrigeration cycle, being the opposite direction in which heat transfer would take place without the application of external power. Common device types include air source heat pumps, ground source heat pumps, water source heat pumps and exhaust air heat pumps. Heat pumps are also often used in district heating systems. The efficiency of a heat pump is expressed as a coefficient of performance (COP), or Seasonal coefficient of performance (SCOP). The higher the number, the more efficient a heat pump is and the less energy it consumes. When used for space heating these devices are typically much more energy efficient than simple electrical resistance heaters.