ENERGY suppliers are failing to explain to customers how to accurately compare tariffs, leaving them potentially unable to choose the best deal, a watchdog has warned.

Which? carried out a mystery shopping exercise by calling 13 suppliers six times each, asking them to explain the Tariff Comparison Rate (TCR), regulator Ofgem's new price comparison tool for consumers.

The TCR is similar to an APR (annual percentage rate) for credit cards and tells customers how much a tariff costs per kilowatt hour based on medium usage, and is intended as a guide to help compare deals.

However, tariffs with the low TCR are not always the cheapest deal, which is why it is important suppliers can clearly explain how it works to customers.

Which? said it received precise information during just four of the calls (five per cent), with only E.On and npower giving an accurate description of the TCR - but then only in two of the six calls they both received.

Richard Lloyd, executive director of Which?, said: "If the energy companies can't even explain how to accurately compare tariffs then their customers stand no chance."

An Ofgem spokesman said: "Ofgem has already told suppliers they must improve their customer service, so they have to make sure their staff can properly explain the tariff com­parison rate to their customers."