Opposing the line in both Massachusetts and Maine regulatory proceedings is NextEra Energy Resources, the world’s largest producer of solar and wind energy, which also has natural gas, nuclear and oil holdings, including oil-fired plants in Yarmouth and South Portland. It also owns a natural gas plant in Massachusetts and a nuclear power plant in New Hampshire.

It’s unclear how much revenue these companies could lose if the line is completed. The companies listed above either did not respond to questions about how the line would impact their revenues or declined to provide any projections.

The production of hydroelectricity produces significantly lower carbon emissions than electricity generated from fossil fuels such as natural gas or oil.

Some renewable energy generators have argued the line will hurt the development of Maine-based solar and wind.

The Maine Renewable Energy Association and RENEW Northeast, nonprofit groups representing companies in Maine’s renewable energy industry, have argued that electricity over the line will cause congestion on the lines leaving the state and make it harder for Maine renewables to sell their energy into New England.

A completed line would run from the Canadian border to Lewiston, where it would plug into the regional power grid. As electricity flows from Maine, which is a net exporter of energy, to the rest of New England, it can hit congestion at several choke points in the southern part of the state. The renewable groups argue the line will make congestion worse and make it difficult for new renewable projects to sell power outside of the state.

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CMP has proposed upgrading capacity at one of those choke points as part of the project, although renewable advocates say that upgrade doesn’t go far enough. As part of its benefits package, CMP has agreed to fund up to $2 million for a study examining options for how to increase capacity at those choke points.

The utilities commission concluded that the renewable groups’ concerns had “little merit.” It also said that making project developers pay to develop space for “future competing projects” on the grid is “poor public policy.”

Three different analyses, including one sponsored by project opponents, have estimated the introduction of hydroelectricity over the line would reduce New England’s carbon emissions by between 3 million and 3.6 million metric tons a year.

That’s the equivalent of taking about 700,000 cars off the road, or roughly 65 percent of all motor vehicles registered in Maine.

However, while the parties agree that the line would reduce Maine and New England’s carbon emissions, project opponents have argued that those reductions could be offset by increases in other regions.

Citing this lack of clarity, Sen. Brownie Carson, D-Harpswell, introduced a bill directing the Maine Department of Environmental Protection to study the issue before deciding whether to grant the project a permit. In the Maine House last week, the bill failed to get the two-thirds majority needed to be implemented immediately as “emergency legislation” and therefore will not become law in time to affect the department’s decision on permitting the line, which is expected to be made in the fall.

Opponents worry Hydro-Quebec doesn’t have enough total energy to fulfill its obligation to Massachusetts and that it will have to divert what it’s currently exporting to other regions to meet Massachusetts’ demand. If that happens, the total amount of hydroelectricity in the world would stay the same and so would global greenhouse gas emissions.

[What you need to know about the CMP transmission line proposed for Maine]

But Hydro-Quebec said it has overbuilt its dam and reservoir system, and in Canada, the company has faced criticism for building more capacity than it can sell. In December, the company said it is actually “spilling” — or wasting — water it can’t convert into electricity because it can’t get that electricity to market without new transmission lines.

Environmental groups are divided on the question of whether the line will decrease global greenhouse gas emissions. The Natural Resources Council of Maine and Sierra Club have argued the electricity will be diverted from other regions, and therefore won’t provide the promised greenhouse gas emission reductions. The Acadia Center and the Conservation Law Foundation disagree, and believe the line will help fight climate change.

The utilities commission agreed with the latter group, writing that construction of the line “will result in reductions in overall [greenhouse gas] emissions through corresponding reductions of fossil fuel generation (primarily natural gas) in the region.”

CMP and Hydro-Quebec have committed to a $260 million benefits package for Mainers that will be paid out over 40 years. When considering inflation, the package in today’s dollars is $72 million, according to the utilities commission.

CMP will cover $230 million of the package, and Hydro-Quebec will pay the final $30 million, according to the agreement.

The heart of the benefits package is a $140 million rate relief fund, which will be used to lower prices for electricity customers in CMP territory. CMP agreed to pay an additional $50 million to lower prices specifically for low-income customers. It also agreed to spend the same amount to lower prices for low-income ratepayers in Massachusetts.

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Projected inflation across the 40-year time frame means the value of the combined $190 million for Maine ratepayer relief will be just $39 million in today’s dollars, according to the utilities commission. The fund’s effects on CMP customers’ monthly electric bills could range from between 9 cents to less than $2 a month, according to varying estimates offered at the commission’s proceedings.

Maine Public Advocate Barry Hobbins described the savings to ratepayers as “obviously, not significant,” during a March utilities commission hearing.

CMP and Hydro-Quebec have agreed to pay $15 million toward increasing the use of electric vehicles in the state, including for building charging stations. The two companies have also agreed to pay $15 million to develop broadband internet access along the transmission line route, and another $15 million to increase Maine’s use of heat pump technology, which extracts heat from outside air to help heat and cool buildings.

Additional funding includes a few million dollars for economic development in Franklin County, education grants and studies related to the Maine power grid.

The project is expected to contribute a total of $589 million to Maine’s economy during the six-year construction period and another $29 million a year after construction is complete.

That’s according to an independent analysis commissioned by the utilities commission, which produced its results in fixed 2009 dollars. The analysis also found the line would generate about $18 million annually in tax revenue for Maine communities located on the line.

While the project is expected to provide some economic benefits to the local economy, Maine electricity regulators acknowledged the line will “have an adverse effect on scenic and property values, and local tourism and recreational opportunities,” in the May order granting a key permit for the line. The order noted these effects “could not be quantified” but would “offset to some degree the economic benefits” of the line.