SSE - Britain's second largest household energy supplier and the parent company of Airtricity in Ireland - lost 28,000 customers across Ireland north and south between March and September.
The Scotland-based company blamed stiffer competition, particularly in the Republic, because of new players.
As of September, it had 782,000 customers in Northern Ireland and the Republic, representing just over a fifth of the market share - down from 810,000 in March.
"The recent decline in customer account numbers reflects the highly competitive market conditions in the Republic of Ireland in particular, which has seen the emergence of new domestic market entrants," the company said in its latest results.
Overall, SSE said it expected its full-year earnings per share to come in at the lower end of expectations because of weak energy prices.
The energy supplier saw an 83pc decline in operating profit in its wholesale division to £26.7m (€33.9m), driven by an 86pc plunge in profit in electricity generation.
The supplier lost 210,000 customers in Britain and Ireland over the first half of its financial year and its users consumed significantly less energy in a warmer-than-average period. The accounts also show SSE paid £35.5m in tax here in the three years to the end of March.