More than 100 unresolved complaints about energy firms have been referred every day on average to an independent watchdog over the last three-and-a-half years, new figures show.
More than 160,000 complaints that could not be settled between firms and their customers have been sent to the Energy Ombudsman in the past 10 years, a rate of 45 per day.
But Ombudsman Services, which runs the independent adjudicator and released the figures to mark the service's decade anniversary, said 130,300 of those grievances had been raised since the start of 2013, some 102 per day.
Chief ombudsman Lewis Shand Smith said: "I'm pleased to see that after all our work there does seem to be a small reduction in complaints this year.
"There is still more work to be done, and we would like to see all companies signing up to the new Energy Switch Guarantee, which should give consumers the confidence to switch suppliers.
"Every year many customers attempting to cut their energy costs suffer problems switching to a cheaper provider, and energy companies who sign up for the service commit to working together to address concerns quickly."
He added: "We are disappointed that only 5% of consumers with an eligible energy complaint come to us for help but looking ahead we will continue to raise awareness of our service through national and local media and outreach activity."
The Energy Ombudsman works with regulator Ofgem, the Department for Energy and Climate Change (Decc) and energy suppliers to improve complaints management and "signposting" across the industry.
It "takes on individual consumer complaints about energy companies which the companies themselves cannot agree upon".
It said it received 14,326 complaints in 2013, rising to 40,776 in 2014, then 54,706 in 2015 and 20,492 so far in 2016, some 130,300 grievances.
The service said 85% of complaints this year had been about billing problems, with 8% stemming from transferring to other providers and sales responsible for 2%, both down on previous years.
One of the so-called big six energy providers, npower, was fined £26 million by regulator Ofgem in December over failures in its billing and handling of complaints.
More than 500,000 customers were affected by npower's billing problems between September 2013 and December 2014, Ofgem said.
In June an energy market consultancy found that growing numbers of dual fuel customers are now with an independent provider rather than one of the Big Six.
Around one in six (17.4%) dual fuel customers across Britain who get their gas and electricity from the same provider were with an independent supplier, consultancy Cornwall Energy found.
It said this marked an increase from the one in eight (12.6%) dual fuel customers who were with independent providers in 2015.
In the last year alone, 2.2 million accounts on any type of tariff were switched to independent suppliers, the research found.