The energy giant reported that domestic electricity customer accounts dropped to 3.46 million in the year to 31 March, down from 3.82 million the previous year, while gas accounts fell by more than 300,000 to 2.32 million.
It came as the group revealed full-year results that "fell well short" of expectations, with underlying pre-tax profits slipping to £725.7 million, a 38 per cent drop from almost £1.2 billion in 2017-18.
The Big Six supplier also pledged to offload its under-pressure energy services arm, appointing a separate board led by executive chair Katie Bickerstaffe to do so.
Bickerstaffe will be tasked with overseeing the separation of SSE Energy Services from the group, "in the form of a listing or new, alternative ownership" by the second half of 2020. She will take up the new role on 23 June.
It also set a deadline for the departure of chairman Richard Gillingwater, who will continue in his position to depart no later than 31 March 2021.
SSE, which called off its merger with Npower late last year, pointed to "persistently high gas prices" as the primary cause of of £284.9m adjusted operating loss incurred in its energy portfolio management arm. This is lower than the forecast of around £300m set out in November, but compares to a £46m profit last year.
However, the group posted a 59 per cent rise in reported profits before tax, which climbed to £1.37bn.
The supplier forecast that tough market conditions and the government's energy price cap will continue to impact earnings into 2019-20.
Chief executive Alistair Phillips-Davies said the group will drive investment in its renewables arm where it "has a large pipeline of opportunities" and will also create a thermal energy business.
It proposed a final dividend of 68.2 pence, making a full-year dividend of 97.5 pence, which it said was in line with its five-year dividend plan.
Gillingwater said: "While our financial results clearly fell well short of what we hoped to achieve at the start of the year, we've made significant progress towards our ambition to be a leading energy company in a low-carbon world.
"We have continued to develop our core businesses of regulated energy networks and renewables; demonstrated our ability to create and unlock value from developing and operating, as well as owning, assets; and adopted clear long-term goals as we set up the business for long-term success.
"The fundamental strengths of our business and the strategic opportunities afforded by the transition to a low-carbon economy will support the delivery of our five-year dividend plan and creation of value for society as a whole."
Peter Earl, head of energy at price comparison website comparethemarket.com, said: “SSE saw an additional 550,000 vote with their feet this year and switch away from the provider, cementing the 430,000 customers it lost to a wave of new entrants to the market 12 months ago.
"It’s evident that the Big Six model is failing to attract customers in the same way that it is used to. Customers are increasingly becoming more aware of excessive price rises, and in turn shopping around for a better deal."