State utility regulators seeking insight into how Central Maine Power handled the backlog of solar energy projects waiting to connect to its distribution grid faced a big reading assignment last month – sifting through 120,000 emails.

That was CMP’s estimate of how many correspondences it would need to compile to answer the initial questions lawyers at the Maine Public Utilities Commission want to know as they begin their investigation into the company’s interconnection practices for clean-energy projects.

Maine’s fast-growing solar industry was in an uproar in February, complaining that CMP was causing unexpected delays and requesting last-minute, multimillion-dollar charges to connect solar farms to the company’s substations. After Gov. Janet Mills called for an investigation, CMP said it had found some technical solutions to do the work faster and cheaper. Since then, CMP’s top management and engineers have instituted ways to slash substation upgrades from as much as $15 million to less than $375,000.

But for the solar industry, many questions remained unanswered.

That led to a petition from the Maine Renewable Energy Association and Coalition for Community Solar Access asking the PUC to formally open an investigation, which the agency did on April 6. Among the issues it will explore: Exactly when did CMP learn the extent of the so-called “over-voltage” problems it faced trying to satisfy hundreds of new interconnection requests? Was its response prudent? If not, should the company face any penalties?

The answers are important. Maine has ambitious goals to fight climate change by encouraging residents to heat homes and drive cars powered by electricity, not oil and gas. Large-scale solar projects will be critical to powering this so-called clean energy economy, and hundreds of projects already have been proposed.

The transformation will take years. But if solar farms can’t hook into the power grid, that’s an immediate problem.

In April, PUC staff lawyers began their process of discovery by asking CMP for specific information. Lots of it. For example:

“Please provide all documents, including but not limited to reports, emails, notes, correspondence, work papers, whether in draft or final form, related to CMP’s identification of and response to the over-voltage issue.”

“I think that would produce between 20,000 and 30,000 emails,” said Carlisle Tuggey, “which is much, I mean, it’s still very large, but more manageable.”

The PUC’s staff attorneys agreed last week to make their request more focused. And after considering CMP’s request to push the date for producing the 20,000 or 30,000 emails to June 16, they compromised on June 2.

This give-and-take isn’t unusual, a PUC spokeswoman said this week. It’s common in a major proceeding for the staff and other parties to cast a wide net and seek numerous documents through the discovery process, Susan Faloon said. If the request becomes too cumbersome and time-consuming, she said, the parties can work together to reach a consensus and refine the document request.

That will still take time. And as is typical in a case like this, no date has been set or even estimated for when it might conclude. That undefined timetable contrasts with the urgency expressed last winter by the solar industry. Faloon was asked if findings produced several months from now would still be useful and relevant.

“We do believe the information will still be relevant when it does conclude,” she said, “as we anticipate renewable interconnections will continue at an accelerated rate for quite some time.”

An industry representative who requested the probe said she’s resigned to the process.

“We want the investigation to be thorough and for the PUC to be satisfied,” said Kaitlin Kelly O’Neill, the Northeast regional director for the solar coalition.

O’Neill noted that solar developers, utility reps, state officials and others have been working this spring to lower the cost of interconnections. Specifically, a working group has been meeting and trying to hammer out a cost-sharing agreement in which all developers hooking up to a certain substation would share the expense of upgrades, not just the first company.

“I’m glad to see there’s no halt to the process while the investigation is ongoing,” she said.

The solar connection issue can be split into two separate parts, said Jeremy Payne, executive director of the state’s renewable energy trade group.

The first is immediate: Find short-term solutions to keep solar developers on schedule. Progress is being made on that part, he said.

The second part is the PUC investigation, which apparently will start by reviewing at least 20,000 internal CMP emails.

It’s important to identify the root causes of last winter’s bottleneck, Payne said. That will help parties understand if it was perhaps limited to actions by a handful of midlevel managers, or whether it represented a systemwide problem that requires regulatory or policy changes.

“I feel like we’re moving in a better direction than a few months ago,” he said, “but there’s still a lot of work to be done.”