Centrica has agreed to buy US firm NewPower Holdings for around £90.95m (€149m).
The acquisition is expected to add around 650,000 customers to Centrica's North American customer base.
NewPower, which is 43% owned by Enron, is the leading retailer of gas and electricity in deregulated US markets.
It serves residential and small business customers in states including Georgia, New Jersey, Ohio, Pennsylvania and Texas.
The deal follows Centrica's acquisition last month of Enbridge Services, which has 1.3 million home and business services customers in Canada.
Centrica has already acquired the Enron Direct business in the UK.
After both transactions are completed, and after the Ontario electricity market opens as scheduled in May, Centrica says it will have around 4.3 million "customer relationships" with North American households.
The UK group says the combined business provides a strengthened platform for further growth in its key target markets, including Texas, Michigan, Ohio and Georgia, and significantly enhances Centrica's market analysis and entry capabilities in other US states.
Centrica's chief executive Roy Gardner says: "We are very pleased to have reached this agreement with NewPower, which helps us achieve critical mass in our target markets in the US.
"This is a tremendous opportunity to combine our sales and marketing skills with the expertise that NewPower brings, and we believe there are significant operational and cost synergies to be gained."