"Not long ago, it took us an average of two days" to complete an installation, says David Thoreau, spokesman for Verengo, which is based in Torrance, Calif. "Now, on a typical house, it takes a crew about five hours."

And Verengo's installation staff has been growing quickly. "In 2010, we had 250" people doing rooftop installations, says Thoreau. "In 2011, we had 450, and at the end of 2012, we had 900."

"We fully expect the rooftop solar industry to continue its current year-to-year growth of 30% for the next five years as the price of electricity continues to increase and the cost of installing home solar continues to drop due, in part, to streamlined permitting and better financing options," says Randy Bishop, Verengo CEO and co-founder.

SolarCity, Verengo and Wall, N.J.-based Trinity Solar have emerged as the leading mass-market installers. According to a recent analysis by GTM Research, SolarCity accounted for 19.5% of residential solar installation in 2012, followed by Verengo with 6.5% and Trinity Solar with 3.8%.

The 17 next-largest installers accounted for a combined 26.1% of the market, and other, smaller installers accounted for the remaining 44.1%, according to GTM Research.

Shake-Ups on the Horizon?

Deep-pocketed players may be looking for more market share in the solar PV rooftop sector, which could pressure some established firms and players.

At the same time, electric utilities, concerned that widespread installation of rooftop solar would reduce demand for utility-supplied electricity, are exploring how to enter the market, as are independent power companies and electricity retailers. Duke Energy, the nation's largest utility and perhaps best known for its large nuclear and coal-fired generation fleets, has been expanding into utility-scale solar power through its Duke Energy Renewables subsidiary and into rooftop solar through Duke Energy Carolinas, its largest unit.

Duke spokesman Jeff Brooks says the utility recently completed the installation of a total of 10 MW of utility-owned rooftop solar systems, many of them on industrial buildings, as part of North Carolina's renewable portfolio standard.

Brooks says laws in many states typically allow only regulated utilities to sell power to customers but forbid entities such as SolarCity to do the same.