Energy firms are sometimes keeping customers on hold for more than half an hour before they can get through – and around a fifth typically take over 10 minutes to pick up, according to Which?
It said companies need to up their game as there are huge variations in waiting times and a lack of consistency on customer service.
The consumer group made 432 calls to 36 energy suppliers to find out how long customers were left waiting before speaking to a member of staff.
On some occasions, it took more than 30 minutes for calls to be answered, the snapshot investigation found.
Each provider was called 12 times at different times of the day and on different days of the week to calculate the average call waiting time for customers.
Which? found that ScottishPower left customers waiting for an average of 21 minutes and 24 seconds before calls were answered – 20 minutes slower than the fastest company in its research, So Energy, which answered calls in just 38 seconds.
Which? said that last year, ScottishPower had been the fastest of the Big Six providers.
This year, EDF Energy was the best of the Big Six, answering calls in three minutes and two seconds on average.
A ScottishPower spokesman said: “Nobody wants to be at the bottom of this type of list and we’re already working hard to regain our top spot.”
The consumer group also found that challenger brand Bulb Energy kept customers on hold for an average of 19 minutes and two seconds before calls were answered, and had the longest single call waiting time of 41 minutes and 48 seconds in its research.
Around a fifth of suppliers included in the research – Avro Energy, Boost, Bulb Energy, Engie, Green Network Energy, Utilita, Utility Point and Scottish Power – kept customers waiting for more than 10 minutes on average.
Twelve of the companies Which? investigated also offered a live chat online for customers to seek help.
Spark Energy was the fastest to respond, taking just 30 seconds on average.
At the other end of the scale, British Gas was the slowest company to respond to live chat queries – with an average wait of seven minutes and 14 seconds.
Which? said British Gas customers may have better luck getting through on the phone – as the energy giant answered calls in four minutes and 19 seconds on average.
When choosing an energy provider, Which? recommends customers consider all factors including value for money and customer service.
Natalie Hitchins, Which? head of home products and services, said: “Energy customers should be able to expect good customer service from their supplier, so it is unacceptable that some people are facing waits of half an hour or more just to speak with an adviser.
“If you are tired of poor customer service and wasted time on the phone, then you should switch to a provider that can better meet your needs, and potentially save you hundreds of pounds.”
Here are the five energy firms with the fastest average response times, according to Which?
1. So Energy, 38 seconds
2. Together Energy, 41 seconds
3. Flow Energy, 45 seconds
4. Ebico, 50 seconds
5. Ampower, 57 seconds
And here are the five slowest average response times, according to Which?:
1. ScottishPower, 21 minutes 24 seconds
2. Bulb Energy, 19 minutes two seconds
3. Utility Point, 18 minutes five seconds
4. Boost, 17 minutes 35 seconds
5. Utilita, 15 minutes 44 seconds