Energy giant SSE, which owns Swalec, has revealed it made a loss of £115.4 million in its retail supply business, just weeks after announcing a hike in household bills.
The company insisted it was battling "difficult" energy market conditions.
The group, which has more than nine million customers and is the UK's second largest generator of electricity, blamed the performance on rising costs of wholesale energy, environmental and social policies and distribution.
Its overall underlying group profits fell 11.7% to £354 million in the six months to September 30.
The first half loss in its retail supply arm compares with a £48.3 million operating profit a year earlier.
SSE was the first of the major suppliers to announce a tariff rise, saying last month that it would lift prices by an average of 8.2% from Friday.