The survey found that just 43 per cent of Trustpower customers were happy with the service they were getting from the company.
"Trustpower's rating was significantly lower than the industry average of 52 per cent. It's the worst score any of the big five power companies have earned in our past three surveys," Jon Duffy, chief executive of Consumer NZ, said.
Second to Trustpower, Contact Energy was ranked among the worst performers, scoring a below-average rating of just 45 per cent happy customers.
Trustpower and Contact Energy have been contacted for comment.
The energy companies with the highest customer satisfaction scores were found to be the smaller operators.
Flick Electric got the highest rating with a satisfaction score of 76 per cent, followed by Nova Energy with 74 per cent and Electric Kiwi with 71 per cent.
The cost of electricity was found to be a major cause of complaint among survey respondents in Consumer's latest power company satisfaction survey.
"For one in three, household power costs were a big worry. Over the past year, 17 per cent said they had trouble paying their power bills," Duffy said.
That figure jumped to 45 per cent among customers of pre-paid retailer Globug.
The survey found that 12 per cent of consumers had overdue fees added to their bill as they could not pay on time.
Duffy said late payment fees were often disguised as "prompt-payment discounts", with the discounts acting as a penalty for consumers who missed paying by the due date.
"Trustpower has a 15 per cent penalty if you miss paying by the due date. That's more than you could be charged by payday lenders, which have interest and fees capped at 0.8 per cent a day," Duffy said.
Four of the five major retailers were found to be using prompt-payment discounts. Only Meridian Energy has removed the use of them.
The Electricity Price Review, which reported to the government in May last year, recommended prompt-payment discounts be banned and that retailers were allowed to only charge reasonable late-payment fees.
Consumer's survey found 16 per cent of consumers missed paying their bill by the due date and that 17 per cent had experienced difficulty paying their bill in the past 12 months.
12 per cent of consumers said they had overdue fees added to their bill, 11 per cent borrowed money from family or friends to pay the bill and 6 per cent had taken out a loan to pay the bill.
