As concerns about climate change and energy security escalate, as government incentives for harnessing solar energy expand, and as these costs decline while those of fossil fuels rise the harnessing of solar energy is expanding on every front . One of the solar technologies that is really beginning to take off is the use of solar thermal collectors to convert sunlight into heat that can be used to warm both water and space.

China, for example, is now home to 27 million rooftop solar water heaters. With nearly 4,000 Chinese companies manufacturing these devices, this relatively simple low-cost technology has leapfrogged into villages that do not yet have electricity. For as little as $200, villagers can have a rooftop solar collector installed and take their first hot shower.

This technology is sweeping China like wildfire, already approaching market saturation in some communities. Beijing plans to boost the current 114 million square meters of rooftop solar collectors for heating water to 300 million by 2020.

The energy harnessed by these installations in China is equal to the electricity generated by 49 coal-fired power plants. Other developing countries such as India and Brazil may also soon see millions of households turning to this inexpensive water heating technology. This leapfrogging into rural areas without an electricity grid is similar to the way cell phones bypassed the traditional fixed-line grid, providing services to millions of people who would still be on waiting lists if they had relied on traditional phone lines. Once the initial installment cost of rooftop solar water heaters is paid, the hot water is essentially free.

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. And, as in China, in some Austrian villages nearly all homes have rooftop collectors. Germany is also forging ahead. Janet Sawin of the Worldwatch Institute notes that some 2 million Germans are now living in homes where water and space are both heated by rooftop solar systems.

Inspired by the rapid adoption of rooftop water and space heaters in Europe in recent years, the European Solar Thermal Industry Federation has established an ambitious goal of 500 million square meters, or 1 square meter of rooftop collector for every European by 2020—a goal slightly greater than the 0.93 square meters per person found today in Cyprus, the world leader. Most installations are projected to be Solar-Combi systems that are engineered to heat both water and space.

Europe’s solar collectors are concentrated in Germany, Austria, and Greece, with France and Spain also beginning to mobilize. Spain’s initiative was boosted by a March 2006 mandate requiring installation of collectors on all new or renovated buildings. Although to be truly effective the mandate needs to be enforced and this has not been the case thus far. Portugal followed quickly with its own mandate. The introduction of government subsidies has also increased the market penetration of solar thermal in Spain. ESTIF estimates that the European Union has a long-term potential of developing 1,200 thermal gigawatts of solar water and space heating, which means that the sun could meet most of Europe’s low-temperature heating needs.

The U.S. rooftop solar water heating industry has historically concentrated on a niche market—selling and marketing 10 million square meters of solar water heaters for swimming pools between 1995 and 2005. Given this base, however, 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, U.S. installation of these systems tripled in 2006 and has continued at a rapid pace since then.

We now have the data to make some global projections. With China setting a goal of 300 million square meters of solar water heating capacity by 2020, and ESTIF’s goal of 500 million square meters for Europe by 2020, a U.S. installation of 300 million square meters by 2020 is certainly within reach given the recently adopted tax incentives. Japan, which now has 7 million square meters of rooftop solar collectors heating water but which imports virtually all its fossil fuels, could easily reach 80 million square meters by 2020.

If China and the European Union achieve their goals and Japan and the United States reach the projected adoptions, they will have a combined total of 1,180 million square meters of water and space heating capacity by 2020. With appropriate assumptions for developing countries other than China, the global total in 2020 could exceed 1.5 billion square meters.

This would give the world a solar thermal capacity by 2020 of 1,100 thermal gigawatts, the equivalent of 690 coal-fired power plants. This would account for more than half of the Earth Policy Institute’s renewable energy heating goal for 2020, part of a massive effort to stabilize our rapidly changing climate by slashing global net carbon emissions 80 percent within the next decade.

The huge projected expansion in solar water and space heating in industrial countries could close some existing coal-fired power plants and reduce natural gas use, as solar water heaters replace electric and gas water heaters. In countries such as China and India, however, solar water heaters will simply reduce the need for new coal-fired power plants.

Solar water and space heaters in Europe and China have a strong economic appeal. On average, in industrial countries these systems pay for themselves from electricity savings in fewer than 10 years. They are also responsive to energy security and climate change concerns.

With the cost of rooftop heating systems declining, particularly in China, many other countries will likely join Israel, Spain, and Portugal in mandating that all new buildings incorporate rooftop solar water heaters. No longer a passing fad, these rooftop appliances are fast entering the mainstream.