NextEra Energy has proposed burying a 68-mile undersea transmission line that would bring much-needed energy into the Boston area, but with a price tag of around $1 billion.

The 520-MW line would run from Salisbury to Lynn, Mass., beginning at the Seabrook generating station.

The NextEra proposal is competing against a cheaper alternative that would run 25 miles of transmission line and make other upgrades to help power Boston, and for $350 million less than than the seabed option.

Burying transmission lines is an expensive proposition, and now NextEra subsidiary New Hampshire Transmission must convince regulators and lawmakers that the added reliability makes the cost worth it. National Grid and Northeast Utilities are partnering on an alternative proposal that would cost substantially less, and both companies have a targeted completion date of 2018.

The Boston Globe spoke with Boston''s chief of environment Brian Swett, who said reliability was a factor in the decision.

“My concern as the energy official in Boston is not only how much my ratepayers are paying but how reliable is that grid — what’s the percentage that it’s on during storms?” Swett told the Globe. “Down time costs my constituents and my taxpayers money.”

NextEra is also arguing that the competing proposal would need to take some active power lines offline during construction and system upgrades, meaning more expensive power will need to be purchased. According to NextEra, the final price tag on the National Grid-Northeast Utilities proposal could be substantially higher than advertised.

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