Power firms keep you hanging on the phone
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.