It now has 5.53 million electricity and 3.47 million gas customers, having doubled the number of consumers contracted to it since the end of 2001, making it the second largest energy supplier in the UK.
Ian Marchant, chief executive of SSE, said the company was "very pleased" to have hit the mark after "seven successive years of growth".
He added: "It is no coincidence that we have achieved this while being independently recognised as the customer service benchmark for the rest of the industry."
Rival energy firm ScottishPower recently announced it had connected the mid-Wales village of Abergeinw to the National Grid for the first time.
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