Dismay over rising energy complaints
Four of the big six energy firms saw complaint levels rise at the start of 2015, according to Citizens Advice.
Complaints to SSE, EDF, British Gas and Npower increased between the final quarter of last year and the first quarter of this year.
Scottish Power had the highest proportion of complaints overall, but saw a slight drop over the turn of the year.
SSE received the fewest complaints, the consumer group said.
Gillian Guy, chief executive of Citizens Advice, said: "The majority of complaints from customers are about their bills, from large back bills to incorrect bills, or even no bill at all.
"With billing a basic part of the service energy firms provide, it should be the simplest thing to get right."
Energy companies said they were working hard and investing in ways to bring complaint levels down.
Both Scottish Power and Npower have been told recently to provide free energy to thousands of customers after they failed to resolve justified complaints to the energy ombudsman on time.