Pacific Gas and Electric Co. passed a renewable energy milestone Thursday when for the first time, rooftop solar panels on homes and businesses could produce 5 percent of the enormous utility’s electricity.

And at that moment, the rules governing new rooftop solar installations in PG&E’s vast territory changed.

Any PG&E customer hooking up a new home solar array must now pay a one-time $145 interconnection fee not faced by prior generations of solar homeowners.

They’ll also pay a new monthly charge — estimated to add 5 percent to their electricity bills — that will help PG&E cover such costs as subsidizing utility bills for low-income families. And they’ll have to switch to “time of use” electricity rates, which charge different prices for electricity at different times of day.

The changes were approved by California regulators in January after a hard-fought battle that pitted utility companies against the state’s growing solar industry. Similar fights have erupted in other states, as utilities and their government regulators grapple with the implications of customers who can now generate much of their own electricity.

In California, the changes tweak a system called net energy metering, or NEM, that compensates solar customers for excess electricity that they export to the grid.

The new monthly charge and interconnection fee kick in whenever the growing amount of rooftop solar power in a utility’s territory reaches a specific threshold, equal to 5 percent of the utility’s peak electricity demand. San Diego Gas and Electric Co. was the first utility to hit that mark, reaching it in June.

PG&E customers whose solar arrays were installed and connected to the grid before Thursday won’t face those new rates and monthly charges — at least not yet. They can continue operating under the original rules for 20 years after their systems were installed.

One key element of the system won’t change: PG&E will continue paying solar homeowners the same rates for their excess electricity, regardless of when their systems were installed.

PG&E, which serves most of Northern and Central California, now has more than 275,000 customers with their own solar arrays. Another 6,000 hook up every month. That’s more than any other utility in the United States, according to the company.

Together, all of those rooftop arrays in PG&E’s territory can generate more than 2.4 gigawatts of electricity, representing almost more than half of the 4.5 gigawatts of rooftop solar installed across California. A gigawatt is roughly the output of a nuclear reactor, though solar panels, unlike reactors, do not produce power around the clock.

“The uptake of solar, the acceleration, has really been remarkable,” said Adam Browning, executive director of the Vote Solar advocacy group. “California, from the Internet to renewable power, has always been ahead of the curve.”

The new charges that solar homeowners will face were designed to offset what PG&E and other utilities consider an unfair subsidy.

As solar customers generate more of their own electricity, they buy less from the utilities. As a result, the utilities’ non-solar customers must shoulder more of the costs associated with maintaining the grid, like fixing substations and power lines.

PG&E has argued that the new system, which the utilities call NEM 2.0, doesn’t solve that problem. But the changes represent a start, according to the company. The California Public Utilities Commission plans to revisit the issue in 2019.

“While NEM 2.0 begins to address those subsidies, we think in California, solar is moving toward a more mature industry,” said PG&E spokeswoman Ari Vanrenen. “At PG&E, we’re committed to finding that right balance.”

Solar companies say the impending switch to NEM 2.0 doesn’t appear to have had much effect on potential customers, though growth has slowed over the course of the year, according to data from Greentech Media.

“We have been selling under the NEM 2.0 rates since early summer, so we have not really seen an uptick in people trying to get in under the old rates,” said Lauren Randall, senior manager of public policy at Sunrun, a San Francisco solar leasing company.

An analysis that Sunrun commissioned found that PG&E customers would still save money by going solar under the new system.

Barry Cinnamon, CEO of Cinnamon Solar, said his Campbell company used the looming change to NEM 2.0 as part of its sales pitch for much of the year, urging potential customers to go solar before the rules changed.

“We were very transparent with customers, and if there was any delay — like they needed to re-roof, or it was a complicated system — we said we couldn’t guarantee they’d get interconnected in time,” he said.

Rich Zbriger was among the last of Cinnamon’s customers to beat the deadline. The 6.1-kilowatt system on the roof of his San Jose home got interconnected this week.

“If we didn’t make it, that would have been OK — I didn’t think it would be that big a deal,” said Zbriger, 70. “But I was pushing Barry to get it done before they hit that magic number.”

David R. Baker is a San Francisco Chronicle staff writer. Email: dbaker@sfchronicle.com Twitter: @DavidBakerSF