Confronting the need to upgrade a 30-year-old system, Central Maine Power hopes to press ahead with plans for the largest transmission project Maine has yet seen.

In the face of $1.5 billion worth of ambition, we are glad regulators are looking closely at the project’s details and assumptions and hope a recent Public Utilities Commission staff report clarifies its essential goals.

The Maine Power Reliability Project seeks to construct 350 miles of new high-voltage transmission lines from Eliot to Orrington, establish new substations and rebuild many miles of existing transmission lines.

There is little reason to doubt CMP’s assessment that improvements are needed for the sake of dependable power. A hypothetical scenario in 2017 showed the clear possibility of an extensive power failure throughout southern Maine under peak summer demand. The company also argues that transmission improvements would encourage the development of wind-energy projects in northern and western Maine, and carry this clean power to the rest of New England.

But a staff report from the Maine Public Utilities Commission is skeptical about some of the arguments and scenarios on behalf of the project. It endorsed just a $1 billion investment for a high-voltage “backbone” to address Maine’s growing need for energy, arguing against several spurs proposed to improve service in regional Maine markets and encourage development of wind power.

The PUC examiners said the requirements of this young industry, as well as a proposal by the Portland firm GridSolar to serve local markets without any transmission upgrade, deserve further study.

Since project costs would be shared by the entire ISO-New England power system, Maine consumers would pay a relatively small share of the overall cost of this project. But as the report noted, regional transmission costs are growing rapidly. To limit the burden on ratepayers throughout New England, regulators must insist on sound and realistic planning.

The examiners’ report makes a strong case for economy, which should be a central focus in the final stage of review. CMP now has an opportunity to respond to staff concerns, and the commissioners of the Public Utilities Commission are scheduled to begin deliberating later this month.

Along with the technical and rate issues raised by the examiners, the commissioners must also weigh the state’s broader needs for economic development and jobs. Finding the right balance in such high-stakes issues will be a difficult assignment.