Of these possibilities, only the humble solar collectors are so far used. they consist of panels, usually mounted on the roof of a building and angled so as to obtain the maximum amount of sunlight. Water is pumped through the panels, picking up heat from the sun as it goes. The pump is controlled by a sensor so that it operates only when the collector is several degrees hotter than the water in the storage tank. Unfortunately, the sun does not shine all the time, particularly in a northern country such as Britain, but despite this it is estimated that a well-designed solar collector system in Britain could halve hot water and space heating . costs by half. In warmer countries the advantages are greater and such collectors are widely used in Israel.