June marks the fourth month in a row Oahu’s rooftop solar industry has seen growth in the number of permits issued compared with the prior year.

The city issued 632 permits in June, up 22 percent from 492 in June 2014, according to Marco Man­gels­dorf, who tracks rooftop solar permits and is president of Hilo-based ProVision Solar.

Mangelsdorf said the primary driver for the growth over the past few months was Hawaiian Electric Co.’s action to clear a backlog of systems that had been waiting for the utility’s approval since October.

The solar industry took a hit when HECO announced in 2013 that all rooftop solar systems needed HECO approval before being connected to the grid.

The number of solar permits issued on Oahu for the first half of 2015 is in line with the same period last year. The city has issued 3,515 permits this year compared with 3,548 issued in the first six months of 2014, said Man­gels­dorf. The value of the projects approved so far this year was approximately $114 million compared with $106 million in 2014.

Solar applications started piling up in 2013 when HECO was slow to approve projects in areas that already had a large number of rooftop systems. HECO said the delay was due to concerns about safety and the stability of the grid if more solar was added in those areas.

Mangelsdorf said he was unsure whether the trend of the past four months would continue, citing recent proposals to change the way rooftop solar owners are paid for the power they send into the grid, a system known as Net Energy Metering.

"To what degree this four-month bump is sustainable remains to be seen," Man­gels­dorf said. "Grid penetration issues and the fate of Net Energy Metering will factor heavily in determining how the PV industry fares during the remainder of 2015."

In a filing with the Public Utilities Commission in June, HECO proposed to increase the minimum monthly bill for new rooftop solar owners to $25 on all islands.

The current minimum bill for all residential customers is $17 on Oahu, $18 on Maui and $20.50 on Hawaii island. In the filing, HECO also proposed to lower the credit new solar customers on Oahu receive to a rate of 18 cents per kilowatt-hour for all solar energy sent to the grid. Currently solar customers are credited at the retail rate for electricity, which averaged 35 cents on Oahu in 2014.

The new charges would not affect current solar customers, who would continue to pay the $17 minimum on Oahu and get the retail rate for solar sent into the grid.

Homeowners are buying solar systems now to take advantage of the current, more favorable rate structure before changes are made.