In the latest annual supplier survey from research firm JD Power and Associates, Npower, which has more than six million customers nationwide and is one of the top three suppliers, came last, while Scottish and Southern Energy came out top of the rankings, having impressed customers with its reaction to last October's storms.
The survey shows that more than half of all electricity and gas customers have chosen to switch their suppliers at least once since the deregulation of the utilities market more than four years ago.
But while customers are exercising their right to shop around and save money, the numbers opting to switch are falling, as is customer satisfaction.
While 51% of consumers have switched supplier in the past four years, the survey showed only 14% had chosen to change this year, a slowdown from previous years.
JD Power polled more than 3000 customers from across the UK and asked them to rank their supplier on six different criteria, including customer service, billing and payment, price and value, reliability, meter reading and supplier image.
Gunda Lapski, director of UK utilities studies at JD Power, said that the difference between suppliers at the top and bottom of the table was increasing and suppliers must do more to satisfy customers.
"Npower has not fared well across all criteria. Suppliers have to address these factors to ensure that the customer is kept satisfied," she said.
An Npower spokesman said he was unconcerned by the survey's findings because he claimed a large chunk of the customer satisfaction criteria focused on electricity distribution, and Npower doesn't have a distribution arm.
"More than a quarter of the survey is based on power reliability so we are affected by things largely beyond our control." However, he admitted that there were "lessons to be learned" from the findings.