'Lottery' of energy customer service
Consumers are facing a "customer service lottery" among energy companies, a charity has said as it called for a wider review of complaint levels.
Citizens Advice said there was a gulf between the best and worst performing energy firms' service to customers.
Scottish Power recorded the highest number of complaints - 944 per 100,000 customers between April and June.
SSE was the best-performing with 47 complaints per 100,000 customers.
Scottish Power blamed teething problems with a new computer system for the high complaint rate, but said it had now taken on more staff to answer calls quicker.
The figures were drawn from complaints made to the Citizens Advice Consumer Service, the Consumer Futures Extra Help Unit, and to Ombudsman Services.
First Utility, Spark Energy, Npower and Co-operative Energy all had more than 400 complaints per 100,000 customers, according to the data.
Seven suppliers had fewer than 100 complaints per 100,000 customers: SSE, EDF Energy, Ecotricity, British Gas, E.On, Good Energy and Green Star Energy.
The Competition and Markets Authority is investigating competition in the energy market. Citizens Advice argued that this should include a review into customer service.
"We have long called for a more competitive energy market and transparency on complaints across the board can help to support that," said Gillian Guy, chief executive of Citizens Advice.
"Consumers need the whole picture on energy suppliers, so they can choose who to buy their energy from on the basis of what is most important to them whether that's price or service.
"Complaints are a significant part of this, and people will rightly question why some energy firms have nearly 20 times as many complaints as others."
Lawrence Slade, of Energy UK, which represents the major energy suppliers, said there were options available for customers who were not happy.
"It is great to see a number of suppliers working hard to put their customers first. However, no one wants to see complaints spiral out of control. Most grievances are dealt with by the end of the next working day with no more than a phone call," he said.
"The key thing here is a range of choice comes into play when shopping around for energy. Customers can consider fuel type, cheaper tariffs and good service when choosing their supplier. Smart meters will also significantly iron out any issues relating to billing when they roll-out in the next few years."