IN THE NINETEENTH CENTURY THERE WAS NO EASY WAY TO heat water. People generally used cookstoves to do it, but first they had to chop wood or lift heavy hods of coal, and then they had to kindle the fuel and stoke the fire. In cities the wealthy heated their water with gas made from coal, but it didn’t burn clean, the heater had to be lit every time they wanted hot water, and if they forgot to extinguish the flame, the tank could blow up. Moreover, in many areas wood or coal or coal gas was expensive and hard to find. To get around these problems, many farmers, prospectors, and other outdoorsmen improvised a safer, easier, and cheaper way to heat water, by painting a metal water tank black and putting it in the sun to absorb as much solar energy as possible. But even on clear, hot days they typically took until early afternoon to get water hot, and they cooled off as soon as the sun went down.

Their shortcomings came to the attention of Clarence Kemp, of Baltimore, and in 1891 he patented a design that had an insulated box around a metal tank with a glass top. Clear glass allowed sunlight in and kept heat from escaping. Kemp’s device, typically mounted on the roof, heated up faster and kept warm longer than any previous design. He called it the Climax. It was the world’s first commercial solar water heater.

He first marketed it to gentlemen whose wives had gone off with their domestic staffs for the summer, advertising that it would make housekeeping easier. But it really took off in Southern California, where fuel was especially expensive and there was a lot of sunshine. By 1897 a third of the residences in Pasadena heated their water with Kemp’s devices.

More than a dozen inventors filed patents that improved on the Climax, but none made a big difference until 1909, when William J. Bailey of the Carnegie Steel Company revolutionized the industry by separating the heater into two parts, an outdoor heating element exposed to the sun and an insulated indoor storage tank to keep water hot through the night. The heating element consisted of a set of long, thin pipes, typically half-inch copper tubing, attached to a black-painted metal backing inside a glass-covered box. This arrangement heated a smaller amount of water faster than earlier designs, in which an entire tankful of water was warmed together. The heated water flowed into the upper part of the storage tank, where it sat above the cold water waiting to be heated and could be drawn from the top.

Soon the company making the Climax was out of business, and between 1909 and 1918 Bailey’s company sold more than 4,000 Day and Night heaters. They worked so well that when a salesman in PaIo Alto set up a heater with a faucet on the sidewalk, people occasionally scalded their hands with it.

Discoveries of natural gas in the Los Angeles basin in the 1920s and 1930s killed the Southern California solar-waterheater industry. William Bailey sold the patent rights for his heater to a Florida firm, and the building boom there in the 1920s, together with high energy costs, created a big business for it. By 1941 the Florida company had installed more than 60,000 units, and over half the population of Miami heated its water with the sun. However, after World War II, as electric rates declined and Florida Power & Light sold electric water heaters at a discount to increase demand, Florida’s solar-waterheater industry withered.

Rising energy prices in the 1970s generated new interest in solar water heating, but when fuel prices dropped in the 1980s and tax credits for buying solar devices dried up, demand fell off once again. Other countries have enthusiastically adopted the Americaninvented solar water heater, though. Today more than 10 million Japanese homes heat their water with the sun. So do 90 percent of Israelis and Cypriots. They’d probably never imagine that the technology came from the Old West.