National Grid is the latest British power company to move into the US market. Last week Scottish Power sealed a $7bn agreement to buy Pacificorp.
National Grid, which owns and operates more than 5,000 km of high voltage transmission lines, has been seeking to diversify and expand its interests outside the regulated power sector in the UK.
David Jones, chief executive, told shareholders last month that the company planned to generate at least a fifth of sales outside the domestic power transmission sector within three years.
He said that the company hoped to complete an American purchase worth at least $1bn by the end of March.
NEES is the holding company for four utilities that provide electricity to more than 1.3 customers in Massachusetts, New Hampshire and Rhode Island. It has a market capitalisation of about $2.6bn.
It is expected that National Grid will pay about $3bn in cash for the company and assume about $1bn of its debt.