Eversource Energy (formerly known as Northeast Utilities) is a publicly traded, Fortune 500 energy company headquartered in Hartford, Connecticut and Boston, Massachusetts, with several regulated subsidiaries offering retail electricity and natural gas service to more than 3.6 million customers in Connecticut, Massachusetts and New Hampshire.

Following its 2012 merger with Boston-based NSTAR, NU has more than 4,270 circuit miles of electric transmission lines, 72,000 pole miles of distribution lines, and 6,459 miles of natural gas pipeline in New England.

On February 2, 2015, Northeast Utilities and all its subsidiaries rebranded themselves as "Eversource Energy". The stock symbol changed on February 19, 2015 from "NU" to "ES".

ES was formed on July 1, 1966, with the merger of Connecticut Light and Power Company (CL&P) (formed in 1917), Western Massachusetts Electric Company (WMECO, formed in 1886), and the Hartford Electric Light Company (formed in 1878) under a single parent company, creating the first new multi-state public utility holding company since the enactment of the Public Utility Holding Company Act of 1935. In 1967, Holyoke Water Power Company (HWP) (formed in 1859) joined the NU System, and in 1992 the Public Service Company of New Hampshire (PSNH, formed in 1926) followed.

PSNH had declared bankruptcy in January, 1988 due to problems getting a license for the completed Seabrook Nuclear Power Plant.

In 1999 Con Edison and Northeast Utilities entered merger negotiations and the companies began preparations to merge, but the deal fell apart in 2001 when Con Edison backed away from the merger after Connecticut's Attorney General Richard Blumenthal threatened lawsuits to block it. The deal would have created one of the largest utilities in the United States.

Legislation passed in the late 1990s deregulated the electricity market in New England and required regulated utilities to divest generating stations to competitive suppliers. In 1999 the company divested all of the generating assets of WMECO and CL&P per requirements of the Massachusetts and Connecticut legislation. The company retained some of these assets by transferring them to a new subsidiary called Northeast Generation which functioned as a competitive supplier and sold the other assets entirely: WMECO's West Springfield Generating Station and several related hydroelectric and fossil fuel generating units were sold to Con Edison, while other assets (most notably the Northfield Mountain hydroelectric facility) were transferred to Northeast Generation.

In 2001, NU sold the assets and operations of its subsidiary, the Holyoke Water Power Company, to the City of Holyoke including the HWP electrical distribution system and customer base and all generation with the exception of the Mt. Tom coal-fired power plant which NU retained. The city's municipal gas and electric department assumed responsibility for the generators and absorbed the HWP distribution customer base. Between 2000 and 2002 due to state laws, NU divested WMECO, CL&P, and PSNH's nuclear generating assets which consisted of their stakes in the Seabrook, Millstone, and Vermont Yankee stations. In 2006, NU decided to sell the generating units it had earlier retained in the 1999 divestiture as competitive suppliers and shutdown its competitive generation business units. The Northeast Generation assets, including Mount Tom Station and Northfield Mountain, were all sold to FirstLight Power Resources. PSNH continued to operate regulated hydroelectric and fossil fuel generation assets to serve its default/basic service customers who did not choose a competitive supplier (the state of New Hampshire had initially not required divestiture of its generation assets), however in 2015 the company (now known as Eversource) agreed to sell all of its New Hampshire generation assets in the same manner it sold its assets in Massachusetts and Connecticut between 2000 and 2006. It began the process of selling off its New Hampshire fleet in 2017.

In November 2005, the company announced it would sell its unregulated competitive businesses, including generation and energy services. In November 2006 the company had essentially completed the divestiture of its competitive businesses.

In October 2010, Northeast Utilities announced that it would merge with NSTAR, with the resulting company retaining the Northeast Utilities name for the next several years. After government approvals, the deal closed in April 2012.

Before its rebranding to the Eversource Energy name in February 2015 the company operated six main subsidiaries: CL&P, PSNH, WMECO, Yankee Gas Services Company (Yankee Gas), NSTAR Electric and NSTAR Gas. All now currently operate under the Eversource name.

Eversource remains Connecticut's largest electric utility, serving more than 1.2 million residential, municipal, commercial and industrial customers in approximately 149 cities and towns. It also serves approximately 200,000 natural gas customers in 71 cities and towns.

Eversource is also New Hampshire's largest electric utility, serving more than 500,000 homes and businesses throughout the state. This subsidiary owns three fossil fuel-fired generating plants and nine hydroelectric facilities, jointly capable of generating more than 1,110 megawatts of electricity.

It is also a main distributor of energy to 1.7 million customers throughout Massachusetts, including more than 1.4 million electric customers in 140 communities and more than 300,000 natural gas customers in 51 communities.

As a large utility company, Eversource and its subsidiaries are regulated by state and federal regulators. The company has contributed to the election campaigns of several Connecticut politicians. Among organizations, the company (known as Northeast Utilities at the time) was the third biggest contributor to Representative Christopher Murphy (CT-5), the fourth biggest contributor to Representative Joe Courtney (CT-2), and the sixth biggest contributor to Representative John Larson (CT-1), during the July 2009 to June 2011 period. During the longer four-year period from July 2007 to June 2011, Northeast Utilities and its executives donated $56,900 to Rep. Christopher Murphy, $38,100 to Rep. Joe Courtney, $30,000 to Rep. John Larson, $11,800 to Rep. Jim Himes (CT-4), and $6,000 to Rep. Rosa DeLauro (CT-3).

In 2015, Eversource fought the rooftop solar industry and supported anti-solar policies. In Massachusetts, they staffed the State House with lobbyists in order to end legislation promoting growth of the solar industry. During the 2015 legislative session in New Hampshire, Eversource opposed an increase to the state's solar net metering cap. New Hampshire's cap is lower than all neighboring states.

As Northeast Utilities the company signed on a joint venture with Hydro-Québec and NSTAR to build a new High-voltage direct current (HVDC) line from Windsor, Quebec (connecting with the Quebec grid) to a location in Franklin, New Hampshire. It is projected that the line will either run in an existing right-of-way adjacent to the HVDC line that runs through New Hampshire, or it will connect to a right-of-way in northern New Hampshire that will run through the White Mountains. This 180- to 190-mile line, projected to carry 1,200 megawatts, will carry electricity to approximately one million homes. The issue of buying hydropower from Hydro-Québec had been an issue during the Massachusetts gubernatorial election of 2010. In November 2015, the Sierra Club of New Hampshire also expressed opposition for the new line, saying that it would not only benefit Connecticut and Massachusetts residents more than those in New Hampshire, but also the concern of the flooding of Boreal forests during the construction of Hydro-Québec's dams in northern Quebec, disputes with the Innu First Nations, and the effects of tourism and the environment within the White Mountain National Forest.

While Eversource reported that its electric transmission earnings were up 80 percent in Q2 2015, the Federal Energy Regulatory Commission (FERC) is now investigating the utility for having transmission rates that appear to be “unjust, unreasonable and unduly discriminatory or preferential”. Meanwhile, the potential for rooftop solar to prevent the need for new transmission lines is growing and Eversource wants to cap rooftop solar growth in the state

In Massachusetts, Attorney General Maura Healey testified in March, 2017 before the DPU urging It to deny Eversource’s proposed $300 Million rate increase. In her testimony, she challenged the need for Eversource’s rate increase, noting NSTAR’s and WMECo’s high returns over the last few years. Referencing NSTAR’s 2015 return of more than 13 percent, Attorney General Healey told the DPU that “[l]ast year, no state public utility commission in the country allowed a return that high.” Between 2010 and 2015, Eversource’s shareholders of common stock received a cumulative total return (including quarterly dividends and the change in the market price per share) of 89 percent.

Eversource Energy has participated in a number of projects to improve the reliability of the power grid in southwest Connecticut. The first project was construction of the $350 million 345 kilovolt Bethel-Norwalk transmission line through the western part of the state, and was constructed entirely by the company when it was still known as Northeast Utilities.

With United Illuminating, an upgrade to the 69-mile (112 km), 345 kilovolt Middletown-Norwalk transmission line was energized in 2009 at a cost of $900 million.

In 2013, the Greater Springfield Reliability Project, a component of the ongoing New England East-West Solution, was energized at a cost of $795 million. The project addressed numerous reliability issues with the Springfield, MA area's 115 kV transmission system by constructing two new 345 kV lines to the Agawam substation; one line north to Ludlow and the other south to North Bloomfield, Connecticut.

The new 345 kV corridor added a new strong interface between Massachusetts and Connecticut. The project also involved rebuilding all of the 115 kV lines along the transmission corridor between South Agawam and Ludlow to increase their capacities, building a new 115 kV transmission substation in East Springfield (Cadwell), replacing the Fairmont 115 kV transmission substation in Chicopee with a new substation across the street, and configuring a new 115 kV line from South Agawam to Southwick using a combination of both new and old line segments of the former 115 kV path between Agawam and North Bloomfield. The new Cadwell and Fairmont switching stations allowed a number of three-terminal 115 kV lines to be broken up into two-terminal lines. Finally, the project allowed a problematic underground 115 kV transmission path through the city of Springfield that was vulnerable to thermal overloads to be removed from service by breaking it in half at the middle. The underground lines now function solely to supply the distribution load served out of the Breckwood substation in Springfield. A previously proposed costly project that would have replaced the underground cables is no longer necessary. On November 20, 2013, cutover of 115 kV lines to the new Fairmont Switching Station was complete, marking substantial completion of the GSRP.

In May 2016, Eversource began an electrical infrastructure project in QueensBrook. The project involves replacing underground wire and transformers throughout the QB.