The plant operated under the jurisdiction of the Selectmen until 1909 when the growing importance of electricity made a separate Municipal Lighting Board necessary. The following year, the Electric Light Department boasted 908 customers, and was more than self-sustaining financially. As technology improved, the arc streetlights were gradually replaced with tungsten incandescent lights, with the last substitution completed in 1911.
In the next few years electric demand increased as consumers learned to enjoy the convenience of such innovations as electric water heaters and stoves. At the same time, sound financial management allowed BELD to proudly become the only plant in the state without debt. And when shortages and high costs made coal impractical to burn, the plant added two new oil-fired boilers in 1921.
Business continued to flourish and expand in the 1920s, requiring the purchase of some power from outside sources. After much discussion and debate, the town voted in March of 1924 to appropriate $50,000 to extend and enlarge the plant. By December of that year, a new generating unit had been installed. Only four years later, Braintree's continued growth forced the plant to buy power from Weymouth Light and Power. In 1932 the plant building was once again improved and enlarged, and the following year a new office building was constructed on Allen Street. In 1939 it was estimated that the plant was worth well over $1 million—a substantial amount in the Depression era. When Ernest Fulton was appointed Manager that same year, the commissioners restated their objectives: "It is the aim of the Commission to furnish current and service at the lowest possible rate and to continue the policy of giving Braintree uninterrupted and economical service."
The 1950s and 1960s saw an unprecedented demand for electricity as the post-War era brought business expansion, a "baby boom," and an explosion of new housing developments. In response to community needs, BELD constructed an annex in 1953 to house another new boiler. At the same time, plans to supply power to the South Shore Plaza required an underground duct system 3½ miles long.