About 230,000 households switched away from SSE in the three months to the end of June.
The supplier raised its dual-fuel prices by 6.9% on 28 April.
Separately, Scottish Power said it had lost 100,000 customers in the first six months of 2017. It raised dual-fuel prices by 7.8% in March.
SSE's figures show an acceleration of customers leaving - after losing 210,000 in the whole of the previous year. It blamed a "highly competitive market".
But Scottish Power, which saw customer numbers drop from 5.4 million in the first half of 2016 to 5.3 million in 2017, said its numbers were stable.
"We have still retained more of our customers over the last five years than any other large supplier," said Keith Anderson, Scottish Power's chief corporate officer.
The figures continue the trend that has seen households leaving the big six suppliers, and moving to smaller companies.
Five of the big six have announced price rises this year - with the exception of British Gas, which promised to freeze prices until August.
The biggest price increase was Npower's. It announced a dual fuel rise of 9.8% in March, leading to calls from the regulator for it to justify such increases.
A third of a million customers have deserted the two big energy suppliers based in Scotland following recent price rises.