The probe is a result of concerns about its approach to switching customers into prepayment tariffs, particularly those in potentially vulnerable situations.
The regulator will examine whether SSE breached its Standards of Conduct which require them to provide appropriate information and ensure consumers’ ability to pay when suggesting alternative payment methods.
An SSE spokesperson said: “We are committed to treating all our customers fairly and will be cooperating fully with Ofgem’s investigation into this historic issue.”
Ofgem’s standards of conduct are aimed at ensuring suppliers treat customers fairly.
In June Ofgem decided to consult on commitments from SSE following an investigation into anti-competitive behaviour in the electricity connections market.