The research also revealed that the organisation's total number of clients, including telecoms and gas boiler maintenance customers, now stands at around 8.4 million.
Speaking about the study, SSE chief executive Ian Marchant, said: "I am delighted that the hard work by teams of people throughout SSE has resulted in us passing the milestone of becoming the UKs second largest energy supplier.
"We want to increase customer numbers further, and will seek to do so through more improvements in customer service, a continued policy of responsible pricing and good product development."
The release of Datamonitor's findings has come after SSE was recently named as the energy supplier with the best customer service record in three separate surveys.
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