the building for storage. One such design was measured lifting the temperature of a house to on a partially sunny winter day (-7 °C or 19 °F), and it is claimed that the system provides passively for the bulk of the building's heating. The home cost $125 per square foot (or 370 m2 at $1,351/m2), similar to the cost of a traditional new home. Active solar heating uses pumps to move air or a liquid from the solar collector into the building or storage area. Applications such as solar air heating and solar water heating typically capture solar heat in panels which can then be used for applications such as space heating and supplementation of residential water heaters. In contrast to photovoltaic panels, which are used to generate electricity, solar heating panels are less expensive and capture a much higher proportion of the sun's energy. Solar heating systems usually require a small supplementary backup heating system, either conventional or renewable. Geothermal heating Geothermal energy is accessed by drilling water or steam wells in a process similar to drilling for oil. Geothermal energy is an enormous, underused heat and power resource that is clean (emits little or no greenhouse gases), reliable (average system availability of 95%), and homegrown (making populations less dependent on oil). The earth absorbs the sun's energy and stores it as heat in the oceans and underground. The ground temperature remains constant at a point of all year round depending on where you live on earth. A geothermal heating system takes advantage of the consistent temperature found below the Earth's surface and uses it to heat and cool buildings. The system is made up of a series of pipes installed underground, connected to pipes in a building. A pump circulates liquid through the circuit. In the winter the fluid in the pipe absorbs the heat of the earth and uses it to heat the building. In the summer the fluid absorbs heat from the building and disposes of it in the earth. Heat pumps Heat pumps use work to move heat from one place to another, and can be used for both heating and air conditioning. Though capital intensive, heat pumps are economical to run and can be powered by renewable electricity. Two common types of heat pump are air source heat pumps (ASHP) and ground-source heat pumps (GSHP), depending on whether heat is transferred from the air