An investigation by the magazine Which? found the response times of the companies’ customer service departments varied widely.
People with nPower, one of the country’s “big six” energy firms, faced the longest average time to wait to speak to an adviser. The company also topped the table for the longest single wait time. Including being transferred across departments, a Which? researcher was kept on hold for 29 minutes.
NPower’s average wait time was 17 minutes and five seconds. This was more than ten minutes longer than the next worst performer – Borders-based Spark Energy.
At the other end of the scale, small energy firm Ebico had the shortest waiting times.
Only six of the suppliers had an average waiting time of less than two minutes, including just one of the big six, EDF Energy.
Which? said no callers should be left hanging on for more than two minutes.
Richard Lloyd, Which? executive director, said: “It’s unacceptable that some energy suppliers are leaving their customers dangling on the phone for anything up to half an hour. If energy companies want to restore trust in the industry, they must work harder to make people feel confident that customer service is a top priority, and stop putting sales over service.
“We’d like to see calls to customer service centres being picked up within two minutes.”
Three in ten people told the magazine that being left on hold for long periods before speaking to someone was one of their biggest bugbears when calling their energy provider’s customer service department.
Other major annoyances included discovering that the phone number they were given to call was not freephone, being forced to go through an automated system before speaking to a real person and having to listen to automated messages being played while on hold.
For the study, Which? called 16 energy companies 12 times each at set times of the day. The watchdog recorded how long it took between finishing dialling the number to getting through to an adviser and then calculated an average waiting time.
Comparing the findings with a previous investigation, Which? found that many firms were slower to pick up calls from existing customers than those from prospective ones.
Of the six major energy providers, all but Eon were quicker to answer calls to their sales number than calls to their customer services.
For those with a non-freephone number, some callers could pay up to 11p from a landline and 40p from a mobile, depending on the company.
A spokesman for nPower said the survey, which was carried out in March, had taken place at a time when the company had closed its call centre for three days to transfer customers to a new system.
Energy companies are said to receive an average 60 million phone calls a year to their customer service departments.