THE number of complaints about Scottish Power has doubled since the beginning of the year, figures suggest.
But the energy complaints league table produced by Citizens Advice and Citizens Advice Scotland shows that Npower still gets more complaints than the rest of the big providers combined.
Both Scottish Power and Npower have seen grievances rocket after introducing new billing systems.
At the other end of the scale, SSE and E.ON had the least complaints among the big providers.
Citizens Advice say Npower received around 648 complaints per 100,000 customers from April to June this year.
That's two and a half times as many again as Scottish Power - who were the next most complained about company - and almost 20 times as many complaints as SSE received.
|Citizens Advice energy complaints figures, April to June 2014|
|Ranking||Supplier||Complaints per 100,000 customers|
The findings come after Ofgem's latest report into the subject showed satisfaction with how the firms dealt with complaints had decreased again.
The watchdog says 57% of customers aren't happy with the way their grievances are dealt with by their suppliers, highlighting three areas of particular concern:
It said energy companies were "frankly awful" at dealing with complaints, and made a fresh call for suppliers to explain what they were doing to improve things.
Its research also showed high levels of dissatisfaction with Npower and Scottish Power.
Npower's customer satisfaction dropped from 36% in the 2012 report to 21% in the 2014 report.
Scottish Power's fall from grace was reflected in satisfaction levels tumbling from 44% to just 20%, the lowest among the Big Six providers.
Meanwhile, just over half of all customers who complained said they felt they had been treated unfairly by their supplier.
It's not surprising that there's a link between the outcome of complaints, and customers deciding to switch supplier.
Again, Npower lead the way for customer service that makes people want to go elsewhere, with 55% of those who complained saying they were planning to leave or already had.
Scottish Power was next, with 24% of complainants saying they'd changed supplier and 25% saying they were going to, a total of 49%.
In contrast, among the other four big providers the numbers ranged from 27% to 40% saying they would move supplier as a result of a complaint and its handling.
Ofgem is already investigating Npower for its customer service.
This time last year the supplier was revealed to be holding £400m in positive balances, and in 2010 it had to pay some £63 million compensation over billing errors.
|Ofgem energy complaints figures, 2014|
|Ranking||Supplier||Customer satisfaction||% felt treated unfairly|
But it's not just the big companies that need to improve customer relations.
Ofgem's latest report included four of the biggest smaller energy companies for the first time.
The report says that 57% of customers who complained about First Utility, Ovo, Co-operative Energy and Utility Warehouse said they felt they'd been treated unfairly.
Among the smaller companies, 55% of customers who complained said they were thinking about switching or already had as a result.
So what can customers do if they do have a problem?
Citizens Advice say it's important to act as soon as possible by contacting the supplier directly.
Recognising the problems they've had this year, Scottish Power have extended their call centre hours to 10pm to help deal with the influx of calls.
As one of the main problems for them and Npower has been the introduction of a new billing system, many customers aren't receiving bills when they expect to.
The advice here is clear: customers should try to keep aside the money they would expect to pay, ready for when the bill does eventually arrive.
Note also that energy companies are only allowed to back bill for energy used in the past 12 months.
That means if it turns out the supplier is at fault, any bills older than a year have to be written off.
Citizens Advice also say people with longstanding complaints shouldn't be afraid to ask for compensation to help cover the time and cost of sorting out the problem.
So suppliers should expect to be asked to help cover the costs of phone calls dealing with the issue, or to recognise and assist with the financial impact of late or incorrect bills - by reducing the balance owed, for example.
In the meantime, energy firms are obliged to take ability to pay into account when setting repayment levels.
And if an issue isn't resolved satisfactorily with the supplier, the next step is going to the energy ombudsman.
Get insider tips and the latest offers in our newsletter
We are independent of all of the products and services we compare.
We order our comparison tables by price or feature and never by referral revenue.
We donate at least 5% of our profits to charity, and we aim to be climate positive.