In the 1760s the Swiss inventor Horace-Bénédict de Saussure made an open wooden box with another inside, put insulation between the two, covered the top with glass, then placed it in the sun. Because glass allows light to pass easily but impedes heat from doing so, the heat was trapped and the box warmed up.
In 1891 Clarence Kemp of Baltimore, USA, patented a combination of this 'hot box' principle with the common practice of exposing metal water tanks to the sun, thereby increasing their capability to collect and retain solar heat. He call his apparatus the 'Climax' – the world's first commercial solar water heater.
In 1909 another American, William J. Bailey, patented a system in two parts: a heating element composed of narrow water pipes exposed to the sun, connected to an insulated tank inside the house, This provided hotter water for longer periods, so Bailey called his system the 'Day and Night', and it revolutionised the business. Solar water systems only made stumbling progress in 20th century but, with the rising costs of conventional fuels, the growing environmental movement and improved technology, homes all over the world are now fast-switching to solar energy.