Oahu and the Big Island are closing in on the total capacity the state allows for residential solar systems that can export excess power to the grid and receive credit.

Oahu is at 70 percent of its total limit, and the Big Island is at 70.6 percent, according to Hawaiian Electric Co. numbers released last week.

Meanwhile, solar applicants on Maui hit the limit in June. Maui Electric Co. said in June it stopped approving the solar systems because the county hit the 5-megawatt cap regulators placed on the program. Maui is at 99 percent of its cap, according to Marco Mangelsdorf, president of ProVision Solar Inc.

The cap comes from a Public Utilities Commission ruling in October. Last fall the PUC replaced a popular solar incentive, known as net energy metering, with a new “grid supply” system and put a 35-megawatt cap on the new program. The grid-supply program credits solar energy customers about 8 cents less per kilowatt-hour than the full retail rate paid under net energy metering.

The cap breaks down to 25 megawatts on Oahu and 5 megawatts each for Maui County (which includes Molokai and Lanai) and the Big Island.

Maui County residents who want to install new solar systems now have to buy batteries to hold any excess power generated by the panels. The only program Maui residents can apply for allows solar owners to draw power from the grid when needed but prevents them from sending additional power to the grid. The solar-plus-battery systems are called “self-supply.”

Both Oahu and the Big Island have jumped 10 percentage points closer to the cap since the last report was filed.

Colin Yost, principal at RevoluSun, said unless the PUC increases the limit, Oahu will likely run out of room by September.

“From the pace of grid supply submissions on Oahu, it looks likely the cap will be reached by the end of August, unless it is raised by the PUC,” Yost said.

In May Yost and a coalition of solar energy groups asked state regulators to increase the limit on excess power that photovoltaic systems are allowed to send to the grid. The Hawaii PV Coalition, the Hawaii Solar Energy Association, Sunpower Corp. and the Alliance for Solar Choice said they will likely fill any remaining capacity on all islands by the end of the summer.