SSE has seen 50,000 customers desert it in favour of rival energy suppliers as it flagged a "volatile" energy market.
The big six player announced in November that it will be freezing household energy prices until at least April 2017, but a third quarter trading update showed that it continued haemorrhaging customers in the three months to December 31, leaving it with a total of 8.08 million.
SSE is under pressure after posting a 13% drop in half-year profits late last year when it admitted to losing another 70,000 accounts.
But SSE has said it is halting the number of customers quitting the group.
On Tuesday it also said that output at its renewable energy plants fell 20% as a result of "dry and still weather".
Chief executive Alistair Phillips-Davies said: "In a changing and challenging energy sector we continue to focus on operational efficiency, disciplined investment and maintaining a balanced range of energy businesses.
"The operating environment presents some challenges. The period since our interim results has featured volatile wholesale energy market conditions and, during November and December in particular, a period of relatively dry and still weather leading to low output of renewable energy."
Shares were down 0.6% to 1,483p in morning trading.
Despite the pledge to freeze prices, experts believe it is only a matter of time before customers will be asked to stomach rises.
Nicholas Hyett, equity analyst at Hargreaves Lansdown, said: "There could be pain ahead for retail customers as well.
"Price pressures are mounting, both as a result of higher energy prices and Government policies, and SSE has only promised to cap prices until April 2017. The group is targeting cost savings, but we still wouldn't be surprised to see prices rise."