nPower has received eight times as many complaints as its closest competitor.
A cloud integrator has warned that energy firms need to improve their back office systems or they will keep receiving customer complaints.
Richard Britton, CEO, CloudSense, issued the warning following the release of the most recent results for energy industry complaints, compiled by energy Ombudsman Consumer Futures.
The study found that complaints against nPower have risen and now stand at eight times the level of its nearest competitor.
This has been attributed to the failed implementation of a new billing system, and Britton described the situation as "disappointing."
He said: "Clearly nPower’s new billing system has resulted in significant problems for customers and as such this issue has the potential to impact nPower’s bottom line.
"Ultimately, receiving a bill is perhaps the only time a customer ever actually interacts with utilities providers, so it’s incredibly important the process is seamless and efficient to avoid negative perceptions and further repercussions for the business, such as a lengthy and expensive investigation and resolution process."
This has become especially important now customers have new and public channels such as social media, through which they can raise issues and highlight problems.
Part of the issue is the inability of enterprise resource planning (ERP) solutions traditionally used by the utilities industry to keep pace with rapid changes in product, pricing and customer information affecting the market, according to Britton.
He added: "To help reduce customer complaints, energy firms need to ensure that they have the right back office systems in place to ensure that bills are right first time to keep customers happy and to avoid costly investigations."
nPower’s director of retail, Roger Hattam, admitted improvements were need, adding: "Last year, we apologised to our customers for the service issues they may have faced, following the installation of a new billing system.
"I know that we’ve let many of our domestic customers down and I want to apologise personally for this and promise that they will not lose out financially as a direct result of these issues.
"We’re working on these issues as top priority and, while we still have a long way to go, we’re making good progress."