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Solar energy projects across much of Massachusetts could hit a stumbling block because of faster-than-expected growth since last summer.

Under state law, utilities can only hook up so many solar energy projects to their systems under the so-called “net metering” cap, which limits the amount of excess power utilities have to buy back from solar installations. In the part of the state served by National Grid, that limit may be hit today or tomorrow, which would force companies in the solar industry to rapidly readjust.

As of 1 p.m. Monday, 99.9 percent of the utility’s net-metering cap has been reached for private-sector solar projects. The public sector installation cap already has been surpassed, with nearly 10 megawatts of solar projects waiting in line. Other utilities are further away from their caps.

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Over the past week, about 20 megawatts of solar power applications were approved in parts of the state served by National Grid, said Ryan Fahey, an employee of the Cadmus Group, Inc. which tracks how much capacity remains under the state’s net metering caps.

If National Grid’s cap is hit, ongoing solar installations won’t exactly come to a grinding halt, but projects in development will be forced to stop. Net metering is one of the first steps for a solar energy project, said Mark Sandeen, a Lexington-based solar developer, so projects under construction already have been approved.

National Grid’s service territory is more rural than the state’s other major electric utility, Eversource Energy, which makes it much easier for solar companies to build big projects, Sandeen said. Unless the cap is raised or suspended, Sandeen said, people who live in National Grid’s territory won’t be able to move their projects forward, and solar power installers could be forced to shift their business into other parts of the state or even lay off workers.

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“If you’re based in Western Massachusetts, this is going to be a tough nut,” Sandeen said.

The impending stoppage stems from a long-running dispute between the solar industry and utility companies over who should pay the cost of transmitting excess solar power back into the electricity grid. Currently, the state has a legally-imposed net metering cap of about 1,000 megawatts of solar power across all utilities, which translates to between 4 and 5 percent of their total generating capacity.

The state Legislature raised the net metering cap last year and formed a task force including representatives from government, the solar power industry, and utility companies. But the members of the task force, which is supposed to submit a report in April, have different ideas about who should foot the bill, said David Colton, the town administrator for Easton. He said municipalities have urged the Legislature to suspend the cap since being alerted to the issue in National Grid’s territory last week.

The Legislature “knew the cap was going to be hit at some point, and as has happened with all of the work that’s happened with solar over the past few years, we’ve continued to exceed expectations,” said Canton, whose town has had to halt the development of a one-megawatt solar development because of the public-sector cap.

Janet Besser, a member of the task force who represents the clean energy sector, said the group was still on target to issue a report by the end of April. In the meantime, she said, the state Legislature could raise the cap “on an interim basis” until it comes up with a permanent solution that would help the state reach its solar goals.

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Several bills have been proposed that would raise or eliminate the net metering cap, but many include other provisions that don’t concern the cap. State Senator Brian A. Joyce of Milton, the assistant majority leader and a member of the Legislature’s energy committee, said the potential solar stoppage had increased the sense of urgency for state lawmakers.

“To let the cap be met would serve no one’s interest whatsoever except the utilities,” he said. “It would be bad for the environment, it would be bad for the economy, and it would be bad for consumers.”

Utilities, however, continue to oppose raising the cap because it passes transmission costs by solar panel owners onto other electricity ratepayers.

“With over 250 [megawatts] still available under the net metering caps statewide, there is no need to raise the net metering caps” while the task force is still deliberating, said Danielle Williamson, a National Grid spokeswoman. “Raising the net metering cap will affect National Grid customers more than other utilities, further increasing rates at a time that our customers are looking for lower energy prices.”


Jack Newsham can be reached at jack.newsham@globe.com. Follow him on Twitter @TheNewsHam.