Energy blls are full of jargon and leave customers struggling to understand how much they owe, a Which? Switch satisfaction survey shows.
When Which? submitted a selection of energy bills to the Plain English Campaign, they were described as being peppered with 'gobbledygook'.
Terms like 'calorific value' and 'normal primary units' were just some of the jargon found on the bills, and confusingly one even used a minus sign for a credit amount.
Around a quarter of Which? members surveyed by the consumer champion said they struggled to understand how much they owe their energy suppliers.
One Npower customer said that he had very little idea how costs were calculated: "I went with Npower because it seemed cheap – but it's so complicated I can't tell if it really is."
The big six energy suppliers received some of the worst customer satisfaction scores Which? has seen overall – even lower than banking providers.
Npower came bottom for the third year running (28%) with British Gas not far behind (38%).
Smaller suppliers fared better, with Utility Warehouse (75%) and Ebico (66%) getting the best customer scores.
Martyn Hocking, Editor, Which? magazine, said: "When you hear the term 'calorific value' you're more likely to think about a diet than your energy bill, but this is the kind of language that’s being bandied about.
"This makes it a struggle for customers to understand their bills.
"Consumers aren't going to be able to reduce their energy use or find the best deal if they don't understand what's going on. Bills need to be much clearer, and better customer service from the energy companies wouldn’t go amiss either," he said.
(CD/BMcC)
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