Of the varying types of geothermal heatpump systems, a borehole is the most independant of climate conditions and is therefore most efficient. It consists of one or more vertical wells (depending on heat demand) into which a collector is sunk. A collector consists of two pipes which draw heat from underground. The advantages of boreholes are that they fit in a smaller garden than a horizontal array. It has a COP (Coefficient of Performance) of about 4.5 kilowatts.