HOLMDEL, N.J. — Jay Nuzzi, a New Jersey state trooper, had put off installing solar panels on his home here for years, deterred by the $70,000 it could cost. Then on a trip to Home Depot, he stumbled across a booth for Roof Diagnostics, which offered him a solar system at a price he couldn’t refuse: free.

Mr. Nuzzi had to sign a 20-year contract to buy electricity generated by the roof panels, which he would not own. But the rates were well below what he was paying to the local utility. “It’s no cost to the homeowner — how do you turn it down?” Mr. Nuzzi said on a recent overcast morning as a crew attached 41 shiny black modules to his roof. “It was a no-brainer.”

Similar deals are being struck with tens of thousands of homeowners and businesses across the country. Installers, often working through big-box chains like Home Depot or Lowe’s, are taking advantage of hefty tax breaks, creative financing techniques and a glut of cheap, Chinese-made panels to make solar power accessible to the mass market for the first time. The number of residential and commercial installations more than doubled over the last two years to 213,957, according to Greentech Media, a research firm.

Major players in the installation business, like SolarCity, Sunrun and Sungevity, are thriving even as the other side of the industry — solar module makers — has been squeezed to the breaking point by fierce competition from Chinese manufacturers. In a case to be decided later this month, a coalition of solar manufacturers has asked the United States government to impose steep duties on the imports, arguing that the Chinese companies are violating international trade rules.