Engineers have also developed a roofing material coated with the electricity-producing film. "The guy who puts on the roof [on a house now] puts on the [solar] panels at the same time," Torcellini said. The roofing material withstands inclement weather and, on bright days, taps sunshine for electricity.
NREL researchers, meanwhile, are working to devise more efficient and cheaper solar-electric systems. Most traditional photovoltaic solar units on the market today convert between 11 and 13 percent of the sun's light into energy. Engineers think they can improve on that.
Jeff Mazer, a Washington, D.C.-area photovoltaic engineer, notes that most thin-film photovoltaic systems today have a 7 to 11 percent efficiency rating. But he estimates that thin films could surpass that rating within three years. He also notes that some new traditional solar modules achieve 15 percent efficiency and believes that figure can climb to 17 percent in the near future.
In the last two decades solar-thermal panels (units used to warm household hot water, pools, and spas) have become highly efficient. Energy costs have decreased from 60 cents to 8 cents per kilowatt-hour since the 1980s, Collins said.
Solar-powered water heaters are typically more expensive than conventional ones, but, as with other products that harness alternative energy, consumers benefit by knowing their energy costs up-front, Torcellini said. "Otherwise, you're hedging your bets about the future cost of [traditional] energy [sources] by using standard appliances," he said.
Compared to other renewable energy sources, wind power competes with conventional energy at a price less than 4 cents per kilowatt-hour, Collins said.
Wind energy projects around the world now generate enough energy to power nine million typical U.S. homes, according to the American Wind Energy Association, a Washington, D.C.-based trade group.
One of the newest trends in wind power is the construction of offshore wind farms, clusters of electricity-generating turbines erected in open-water areas with strong winds.
Europe now has 17 wind farms spinning offshore. The Arklow Bank Offshore Wind Park, 8 miles (13 kilometers) off the eastern coast of Ireland, is one such project. Its seven turbines generate enough electricity to power 16,000 homes.
While few homes generate their own wind power in the U.S., many power companies allow consumers to opt for power generated at a wind plant or other renewable source.
On Tuesday, Colorado voters will consider a ballot initiative that would require power companies to provide 10 percent of their electricity from wind and other renewable sources by 2015.
"If that passes, power companies will offer more rebates to homeowners" to encourage renewable energy production, said Sheila Hayter, an NREL senior engineer.
Tapping into the ground offers another option to regulate household heating and cooling.
In most areas of the United States, the ground below the frost line maintains an average temperature between 50 and 54 degrees Fahrenheit (10 to 12 degrees Celsius).
Ground-source heat pumps, also called geo-exchange systems, use this relatively constant temperature to keep homes at comfortable temperatures.
The devices employ a series of underground, liquid-filled tubes or wells. Liquid flows through the pipes into the home, where a heat exchanger either adds or subtracts heat from indoor air, depending on the season. In winter, that means added warmth captured from the ground.
"If you can [do that], your furnace doesn't have to work so hard," Hayter said.
A U.S. Environmental Protection Agency study found that geo-exchange systems can save up to 70 percent of home-heating costs.
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