Airtricity's parent company SSE lost 28,000 customers across the island of Ireland between March and September.
The Scotland-based company blamed stiffer competition, particularly in the Republic, because of new players in the market.
As of September, it had 782,000 customers on the island of Ireland - just over a fifth of the market share.
SSE also said it expected its full-year earnings per share to come in at the lower end of expectations because of weak energy prices.