Photovoltaic building permits issued by the city in September fell for the second straight month to bring their decline to 48.2 percent through the first three quarters from the year-earlier ­period.

The number of permits on Oahu last month fell 33.5 percent to 252 from 379 in the same month last year, according to recent data from Marco Mangelsdorf, who tracks rooftop solar permits and is president of Hilo-based ProVision Solar.

Through the first nine months of the year, the city Department of Planning and Permitting issued 1,963 PV permits compared with 3,789 over the same period last year.

Solar permit numbers have been falling since the state Public Utilities Commission ended a popular ­solar incentive program in 2015.

The program, called net energy metering, or NEM, credited customers the full retail rate for the excess energy sent to the grid. One less-lucrative replacement called grid-supply, which has a limit on the number of participants, credits customers about 15 cents a kilowatt-­hour.

Once the limit is hit on grid-supply, the remaining program for customers looking to have a solar system — self-supply — encourages customers to purchase batteries.

“We in the solar electric industry have another handful of days left before the customer grid-supply program disappears for good,” Mangelsdorf said. “Other than customer self-supply, what comes next? All eyes are on the Public Utilities Commission for the answer to that question.”

Mangelsdorf said the same question can be posed next month when the U.S. International Trade Commission recommends trade remedies to President Donald Trump to provide relief to existing American PV module assemblers that have been hurt by cheap ­solar panel imports.

“In the meantime, there’s substantial cloudiness as to how much we’ll be paying for new solar module orders, making it hard to price sales quotes and close deals with that degree of uncertainty,” Mangelsdorf said.

On the energy storage front, Mangelsdorf said there were 349 PV permits including batteries issued from May through September, with the number peaking at 97 in July. He said during that period there were 18 permits issued for batteries only, most likely for existing grid-tie solar customers wishing to add storage.