Some of the UK's biggest energy suppliers are currently among the best for handling customers' complaints.
A Citizens Advice league table shows that SSE had the lowest level of complaints, at just 22.5 per 100,000 customers.
EDF, British Gas and E.On took second, fourth and fifth spot respectively.
But big rivals Npower and Scottish Power came near the bottom of the league table, compiled for the April to June quarter this year.
A small supplier, Extra Energy, was worst of all the 21 firms measured, with a complaints rate that was 80 times greater than that of SSE.
The firm, which launched in 2014, apologised to customers and said it was now dealing with more complaints by its target of the end of the next working day.
"These figures reflect historic customer service issues that occurred during a period of time where Extra Energy saw our number of customers expand rapidly and unfortunately some of these complaints have taken longer than expected to resolve," said Ben Jones, the firm's managing director of operations.
Complaints are typically about late or inaccurate bills, or the difficulty some customers experience while simply trying to contact their gas or electricity supplier.
Citizens Advice said the performance of firms in dealing with complaints had fluctuated considerably in the first six months of the year.
"The latest league table shows some suppliers are getting much better at sorting out their customers' problems, but it's disappointing to see others getting worse at dealing with complaints," said its chief executive, Gillian Guy.
"Patchy and unpredictable customer service affects people in many ways - from the financial stress of being hit with a late bill, to the time wasted trying to get hold of a supplier on the phone."
The table shows that the overall gap between the best and worst firms was at its widest since Citizens Advice first started publishing its league table five years ago.
The results reveal that:
- SSE was at the top with an improving complaints rate of 22.5 per 100,000
- Extra Energy stayed at the bottom of the table for the second quarter in a row, with a complaints rate of 1,791 per 100,000
- Npower, notoriously bad at dealing with complaints in recent years, saw its complaints ratio improve by a third - down from 653 per 100,000 to 383 per 100,000
Claire Osborne, at the price comparison service uSwitch, said the latest results were encouraging, but some firms still had "work to do".
"Inaccurate bills, one of the main causes for complaints, are unfortunately much more common than we would like," she said.
"Our own research found that almost four million customers have been overcharged due to billing errors over the last year - leaving them £270m out of pocket.
"This data shows that there is more to choosing an energy supplier than price alone, so it's important to do your research before choosing a provider," she added.
Small Business, Consumers, and Corporate Responsibility Minister Margot James said: "Poor customer service can erode trust and cause unnecessary stress for bill payers.
"It's great to see that some companies are improving, but all energy suppliers should be treating their customers fairly and ensuring that any complaints are suitably addressed. If they don't, their customers will vote with their feet and switch to a different supplier."
If you have an unresolved complaint about a gas or electricity company, the Energy Ombudsman can help.
Click here for more information.