The government tests cars for gas mileage. Now it's testing roof tiles for wattage.

Around 500 square feet of PV tiles can produce three kilowatts of electricity, according to Subhendu Guha, president and chief operating officer of United Solar Ovonic, a maker of PV shingles in Auburn Hills, MI -- and most roofs are several times that size. His company's version is dark blue and can blend with ordinary shingles of a similar shade. Or a builder might devote an entire sunny section to PV materials.

"A south-facing roof on a three-bedroom home could supply 20 to 30 percent of the home's electrical needs," says Paul Maycock, a consultant and head of PV Energy Systems in Williamsburg, VA.