Gas bills can confuse customers, Which? says
Energy bills are filled with jargon such as "calorific value" and "normal primary units" that make them tough to understand, campaigners say.
The language used by energy companies made it difficult for customers to understand how much they owed, the consumers' association Which? said.
It wants summary boxes on bills to outline key information.
But the body that represents the energy suppliers said that bills were being made clearer.
The Energy Retail Association (ERA) said that customers would be receiving an annual statement on their energy use by the end of next year.
"Energy companies issue over 200 million bills each year and suppliers continually work to improve the clarity of information to customers," said ERA's chief executive Garry Felgate.
The survey of nearly 4,000 Which? members found that gas and electricity bills were more difficult to understand than other household bills, such as credit card and mobile phone demands.
kWh: Kilowatt hours - the standard measurement on an energy bill that shows your usage
Calorific value: The quality of gas, showing the amount of energy created from burning the gas
Primary/secondary units: You are billed more for primary units, and move on to cheaper secondary units when these are used up
mpan: The 21-digit unique electricity meter point administration number
mprn: Unique gas meter number, irrespective of your supplier
The Plain English Campaign found clarity problems regarding all eight suppliers' bills it was asked to check, describing some of the language as "gobbledygook".
One bill used a minus sign to denote that a customer was in credit, and terms such as "calorific value" - the quality of the gas a customer receives - were not widely understood.
Which? described the use of this language as "insulting" to customers who needed to understand their bill for financial and environmental reasons.
"When you hear the term 'calorific value' you are more likely to think about a diet than your energy bill, but this is the kind of language that is being bandied about. This makes it a struggle for customers to understand their bills," said Martyn Hocking, editor of Which? magazine.
"Consumers are not going to be able to reduce their energy use or find the best deal if they do not understand what is going on."
Mark Todd, of switching site Energyhelpline, said most people just looked at the amount on the bottom of their bill to see if it was changing month to month.
"If people do not understand their bill then the whole energy supplier situation is out of control for consumers," he said.
"They are hard to compare and people do not know if they are getting the correct bill or not."