We don't often hear about solar water heating, but it's taking off around the world.

Unlike solar PV, which convert solar radiation into electricity, solar thermal collectors use the sun's energy to heat water, indoor space, or both.

Guess which country leads on solar water installations? Yes, China has nearly two-thirds of the world total, with an estimated 1.8 billion square feet at the end of 2010.

That's the equivalent of 118,000 thermal megawatts of capacity, enough to supply 112 million Chinese households with hot water.

With some 5,000 Chinese companies manufacturing these devices, this relatively simple low-cost technology has leapfrogged into villages that don't yet have electricity.

For as little as $200, villagers can install a rooftop solar collector and take their first hot shower. The technology is sweeping China like wildfire, already approaching market saturation in some communities.

Beijing's goal is to reach 300 million square meters of rooftop solar water heating capacity across the country by 2020, a goal it is likely to exceed.

Other developing countries such as India and Brazil may also soon see millions of households turning to this inexpensive water heating technology.

In Europe, where energy costs are relatively high, rooftop solar water heaters are also spreading fast. In Austria, 15 percent of all households now rely on them for hot water. Germany is also forging ahead. Some 2 million Germans have rooftop solar systems. Roughly 30 percent of the installed solar thermal capacity in these two countries consists of "solar combi-systems" that are engineered to heat both water and space.

The U.S. rooftop solar water heating industry has historically concentrated on a niche market -- selling and marketing more than 9 million square meters of solar water heaters for swimming pools between 1995 and 2005. Given this base, the industry was poised to mass-market residential solar water and space heating systems when federal tax credits were introduced in 2006. Led by Hawaii, California, and Florida, annual U.S. installations of these systems have more than tripled since 2005.

Despite the recent growth in US installations, the country ranks 36th in installed capacity relative to its population, with just 0.01 square meters installed per person.

Cyprus leads the world in solar water heater area on a per capita basis, with 0.79 square meters per person. Israel ranks second with 0.56 square meters per person.

Inspired by the rapid adoption of rooftop water and space heaters in Europe in recent years, the European Solar Thermal Industry Federation (ESTIF) established an ambitious goal of one square meter of rooftop collector for every European by 2020. Over the long term, they estimate solar thermal has the potential to meet most of the region's low-temperature heating needs.

Numerous policies promoting renewable energy use for water and space heating exist around the world. Some governments have gone a step further, passing laws requiring solar water heaters in new construction.

For a quarter-century, Israel was the only country to have a national mandate for solar hot water in buildings. Then in 2006, Spain began requiring that solar collectors be installed on all new or renovated buildings. Portugal followed quickly with its own mandate. In the US, Hawaii now requires all new single-family homes have them.

For more data and information on the rapid growth of renewable energy worldwide, see World on the Edge by Lester Brown.

Website: www.earth-policy.org

Photo by gmourits/flickr/Creative Commons