California cities are leading the nation in eliminating one of the biggest hurdles to the growth of residential solar: lengthy and confusing permitting.

Spurred by a recent state law, hundreds of California communities have streamlined their permit process for small residential solar systems over the past year, some bringing it down to a single day. Some cities have also fast-tracked inspections to within a few days of permit approvals. The outcome? The state's biggest cities are now processing and signing off on hundreds of these solar projects each month.

San Jose, for example, streamlined its permit review and approval process last August and has since approved more than 4,500 residential rooftop solar permits. That's a nearly 600 percent increase over the previous year, when San Jose, California's third-largest city, permitted a mere 661.

"We want to make it fast and easy so that no one can hesitate due to bureaucratic red tape to be able to make the transition to a greener energy source," San Jose Mayor Sam Liccardo told InsideClimate News.

In 2014, California legislature's passed solar bill AB 2188, called "The Expedited Solar Permitting" Act, which gave cities and counties in the state a September 2015 deadline to establish a fast, streamlined permitting process for residential solar systems of 10 kilowatts or smaller. California is the first and possibly the only state to require expedited permitting.

The rapid permitting and inspection process dramatically cuts the so-called "soft costs" associated with solar energy. Soft costs account for more than half of residential solar costs, according to a 2013 study by the Department of Energy's Lawrence Berkeley National Laboratory. The study also found that different permitting practices "can have a meaningful impact" on solar prices.

While the prices of residential solar hardware dropped by over 14 percent between 2014 and 2015, soft costs actually increased by about 6 percent, according to Greentech Media Research.

Permits for an average residential rooftop solar system with a capacity of 10 kilowatts (kW) or smaller cost about $310 in San Jose. Customers can now save $40 by getting the permits online, according to Bill Mayne, San Jose's division manager of inspection services. The new process is also saving the city, solar companies and contractors and homeowners money by reducing the man-hours dedicated to installing a solar system.

The average California home uses about 6,700 kilowatt-hours (kWh) of electricity a year (far below the U.S. average of 10,932), according to the U.S. Energy Information Administration's data for 2014. A 10 kW solar system in San Jose will more than cover the needs of an average home there, generating around 15,600 kWh per year, according to PVWATTS Calculator, an online tool run by the National Renewable Energy Laboratory.

"When cities streamline the solar permitting process, consumers win," said Alex Hobson, a spokeswoman at the trade group Solar Energy Industries Association (SEIA). "The reduction of soft costs makes going solar quicker and even more cost-effective."

SEIA interim president Tom Kimbis has previously pointed to San Jose's one-day solar permitting process as a model for the nation, according to Fast Company.

"San Jose's overall process is by far one of the best in California," said Danielle Bentz, SolarCity deputy director of policy. "Permits are processed within minutes via their online portal and inspections are scheduled in batches, sometimes on Saturdays, unlike any other jurisdiction."

"It's much easier now than when I put solar on my house, about a decade ago," said Liccardo. "I suffered mightily." He said that experience played into his support for the city's rapid permitting process and the other areas in which the city is pushing ahead on clean energy.

"We have opened our city to become a laboratory, or perhaps less eloquently, a guinea pig," Liccardo said, citing a new pilot program by the utility Pacific Gas & Electric and SolarCity. That program will test solar inverters, tools to efficiently feed unused solar panel energy back into the grid, as well as storing some for later use. The city has also streamlined its permitting for electric vehicles.

San Jose, home to about 1 million people and located in the heart of Silicon Valley, ranked fifth among American cities in total installed solar power by the end of 2015, according to a report released this spring by the green group Environment California. (The four top cities, in order: Los Angeles, San Diego, Phoenix and Honolulu).

San Jose was among the earliest to meet California's new fast-tracking requirements, with officials unanimously passing the ordinance in August 2015. The city immediately tested its new online program for filling out permit applications, which are immediately processed and available to print out at home with inspections possible the same day. The program officially launched the following month.

"The first few months, we were behind and trying to play catch up," explained Mayne, one of the driving forces behind the new program. "Now a year in and we are on the other side."

The city tackled the backlog with an army of trained inspectors. In small classes, inspectors are taught the basics of solar power, walked through what proper systems look like and ride along with trained inspectors on multiple project reviews and inspections before graduating.

The city increased its number of trained inspectors from 10 to 35 and 20 more will likely be trained by the year's end.

"That's great," said Nick Hylla, executive director of the Midwest Renewable Energy Association, a clean energy advocacy group. "We deal with a lot of jurisdictions that have only one inspector."

At least 386 of California's 540 jurisdictions have already adopted streamlined permitting, and many more are still developing their process, according to a recently released review conducted by the University of San Diego School of Law's Energy Policy Initiatives Center. Not all of the cities have same-day permits and inspections, but San Jose is far from alone.

The nearby city of Milpitas (pop. 78,000) adopted an online process more than a year before the California mandate was put in place. So far this year, the city has permitted 64 small residential systems. Its permits cost only $141, but the demand is far lower compared to San Jose's, and the city has only four trained solar inspectors on staff.

Los Angeles is another early adopter. It established an online permitting process for small residential systems in late 2014 and has a 24-hour turnaround on inspections.

The city processes over 100 small residential permits a week, according to L.A. official Osama Younan. Similar to San Jose, L.A. has trained all of its inspectors in residential solar systems and has 100 inspectors on staff—nearly three times the trained staff of San Jose. But as the state's largest city, it also has a population of about 4 million people.

But despite Los Angeles's clear advantage in resources, San Jose is gaining on the Southern California solar powerhouse—processing nearly as many applications and boasting a far greater amount of installed solar per capita.

"We know other cities are still out in front of us and we expect to get out and catch them and pass them because that's how we roll in Silicon Valley," said Liccardo.

Clarification: A previous version of this story illustrated how much electricity a residential solar system in San Jose could produce a year by comparing it to the average electricity use of a U.S. home. While this comparison correctly showed that the solar system will likely more than cover the home's electricity needs, it did not correctly demonstrate the magnitude of the difference. The story has been updated to include how much electricity a typical California home uses a year to more accurately reflect just how much extra electricity a 10-kilowatt residential solar system in San Jose will likely produce.