Many California and Bay Area [solar] companies are in a period of explosive growth. Companies such as SolarCity, Sungevity, SunPower and Sunrun are installing panels at a heady pace, and adding jobs along the way. Their expansion has been fueled by ... a worldwide plunge in the price of solar cells. Companies that design and install solar systems for homes, businesses or utilities have seen their sales rise. They're not just survivors - they're strong survivors," said Lyndon Rive, chief executive officer of SolarCity in San Mateo. "And it's not just us. It's the industry. ... The notion that it's a failure is so outrageous." The number of solar installations - both large and small-scale - is booming. In 2013, the United States added enough new photovoltaic panels to generate a maximum of 4.2 gigawatts of electricity, roughly the output of four nuclear reactors. Over the past five years, the number of residential installations has grown at an average annual rate of 70 percent, according to the NPD Solarbuzz market information firm. "The demand today is coming from the fact that someone can put solar on their house and save money," said Paul Nahi, CEO of Enphase Energy, a Petaluma company that makes microinverters for solar arrays. "It is true that they may also be saving the planet. But that's not their main consideration." The drop in prices isn't their only reason for growth. Companies including SolarCity, SunEdison and Sunrun began offering solar leases or power purchase agreements to homeowners and businesses. Rather than buy the panels, customers could just buy the energy. That financial innovation revolutionized the industry.