Oahu’s solar industry continued to shrink in 2017 as the number of building permits issued for rooftop solar projects by the City and County of Honolulu declined nearly 35 percent from the year-earlier period.

The largest solar electric market in the state had 2,993 solar permits last year compared with 4,591 in 2016 and 7,493 in 2015, according to Marco Mangelsdorf, who tracks rooftop solar permits and is president of Hilo-­based ProVision Solar. The peak for photovoltaic permits issued on Oahu occurred in 2012 with 16,715, leaving last year’s number down 82 percent from the all-time high.

Oahu’s 2017 drop in permits exceeded the 27.9 percent decline on Hawaii island but was less than Maui’s 59 percent.

In December, Oahu solar permits issued fell just 3 percent to 350 from 361 in December 2016. Of the 723 solar permits issued for November and December, 272 of them, or about 38 percent of that total, included batteries.

Mangelsdorf said two new interconnect programs — Customer Grid Supply Plus and Smart Export — approved in October by the state Public Utilities Commission are in the pipeline this year and would account for a combined 84 megawatts worth of capacity across Hawaiian Electric Co. service territories.

“The timeliness of the actual rollout of these new programs gives some of us in the industry concern, though, as some of the technological challenges to actual implementation, as far as new systems going online, are not trivial,” he said.

Under the Customer Grid Supply Plus program, customers can install solar systems that aren’t connected to storage batteries, and receive electrical bill credits for power exported to the utility during the day. However, these customers will have to have advanced equipment that allows HECO to reduce what is sent to the utility through its grid to prevent any problems with too much energy flowing through the grid.

The PUC said “CGS+” may be available to about 5,000 to 6,000 customers on Oahu, Hawaii island, Maui, Molokai and Lanai. CGS is the name of an older program that has reached capacity, but HECO is still accepting applications in case those who already were approved decide to withdraw their applications. CGS credits Oahu participants 15 cents a kilowatt-hour for any excess energy they send to the utility. An earlier and now closed program where customers are paid the full retail rate, or around 25 cents a kilowatt-hour on Oahu, was called net energy metering.

The rate paid to CGS+ customers will be 10.1 cents on Oahu, 10.6 cents on Hawaii island, 12.2 cents on Maui, 16.8 cents on Molokai and 20.8 cents on Lanai.

With “Smart Export,” customers who install solar systems with batteries have an option to export power during nondaylight hours (4 p.m. to 9 a.m.). Typically, such systems generate power during the day, and this power is largely stored for customer use. This export credit is 15 cents on Oahu, 11 cents on Hawaii island, 14.4 cents on Maui, 16.6 cents on Molokai and 20.8 cents on Lanai.

This program may be available to roughly 3,500 to 4,500 customers, the PUC said.

Mangelsdorf said “another ray of sunlight this year” will be the accelerating deployment of homeowner-side-of-the-meter energy storage.

“With a potential new customer base of more than 80,000 existing grid-tie PV systems across the islands, the vast majority of them with no battery storage, and perhaps a more acute feeling of vulnerability in light of the devastation of other island states such as Puerto Rico, the addition of batteries to even a small initial percentage of that base holds promise,” he said.