Complaints about energy suppliers increased by 23% in 2015.
They accounted for 65,168 compared to 52,937 in 2014, according to the energy Ombudsman, which resolves disputes between customers and energy firms.
However energy firms are starting to show signs of improvement, it added.
Complaints in the last three months of the year fell by 29%.
Billing is still a common problem for consumers, accounting for 83% of the complaints received.
Transfers issues like switching suppliers or tariffs are also a source of bother for consumers, accounting for one in 10 complaints received, it stated.
Chief Ombudsman Lewis Shand Smith said: “Energy complaints rose by nearly a quarter over the course of last year as customers continue to be more vocal about their discontent with suppliers.
“Towards the end of the year, we’ve seen some suppliers take some encouraging steps, particularly when it comes to improving their billing processes but there’s still more that can be done. We encourage anyone experiencing an ongoing issue to speak to us – be it for advice or resolution, we’re here to help and our complaints process is easy to use and free to consumers.”
A survey by Which? revealed the Big Six energy suppliers are still providing a “poor level” of customer service.