Around 6.5 million complaints were made by customers to the larger energy suppliers last year.
That’s according to latest figures from Ofgem, which revealed the main issues were bills, customer service, metering and switching.
The regulator also found 1.5 million electricity and 1.4 million gas domestic accounts were in debt since the beginning of 2014. The average level was £323 for gas and £306 for electricity.
Between January and December 2013, 84 domestic gas and 556 electricity accounts were disconnected as a result of debts.
Distribution network operators have paid £6.9 million since 2013 for failing to meet standards while energy firms have paid more than £160 million in compensation and fines since 2010.
Ofgem said: “Our Simpler, Clearer, Fairer reforms have introduced tough new standards and enhanced transparency to help make sure consumers receive fairer treatment. We’ve also won new powers to make suppliers compensate customers directly when standards aren’t met.”
Earlier this month, it was revealed ScottishPower received the highest number of complaints ever recorded for a UK energy supplier.