A basic use of solar energy is for heating the fluids of air and water. For instance, houses in cold and temperate climates often need heated air for comfort, and in all countries hot water is beneficial for personal and clothes washing as well as for other domestic purposes. There are similar needs in business, industry, and agriculture. Consequently, considering national energy supply, in the UK about 30% is used for such heating within buildings, and even in Australia with a warmer climate about 20% is used for heating fluids, predominantly water, to ‘low’ temperatures (<100°C). For solar energy systems, if the insolation is absorbed and utilized without significant mechanical input (e.g. for pumping or blowing), the solar system is said to be passive. If the solar heat is collected in a fluid, usually water or air, which is then moved by pumps or fans for use or storage elsewhere, the solar system is said to be active.