Complaints by household energy customers fell by a third last year to their lowest point in three years, figures show.
The total number of complaints by domestic consumers dropped from around five million in 2015 to 3.5 million last year, or by 32%, according to data published by Ofgem and the Energy Ombudsman.
Ofgem chief executive Dermot Nolan said: "Ofgem welcomes the overall fall in complaints since 2014.
"We are seeing clear signs that some suppliers are competing harder on customer service. This is good, as it puts pressure on poor-performing companies to up their game or lose market share.
"We want to see all suppliers take steps to bring down complaints further. We take strict action where we see companies failing and have imposed over £50 million penalties for companies for poor customer service."
Chief Ombudsman Lewis Shand Smith said: "The steady decrease in the volume of complaints over the last year is encouraging for the energy industry as it indicates that providers are improving their service and complaints handling.
"It's clear that energy companies' steps to improve their customer service are starting to pay off, but there is still more that could be done to ensure the service customers receive matches their expectations."
Separate Ofgem figures earlier this week showed that customer switching between energy suppliers has surged to a six-year high as householders increasingly shop around for better deals.
Lawrence Slade, chief executive of trade body Energy UK, said: "Complaints falling by a third last year, to a three-year low, shows the level of commitment and investment by the industry to making improvements to customer service.
"However, there is no room for complacency and energy companies are committed to continuing to raise satisfaction and putting their customers first."