Utility Warehouse was this year’s top-scoring energy supplier, according to the survey of nearly 9,000 customers, while all the Big Six energy firms were in the bottom third of the tables. Just a third of their customers (32%) said they are very satisfied, compared with 52% of medium energy companies’ customers and 45% of small suppliers’ customers.

Flow Energy, Octopus Energy and PFP Energy also scored well. Which? said the best companies tended to score highly with customers on value for money and accuracy and clarity of bills. Price remains the most important motivation for switching.

Which? highlighted Utility Warehouse’s unique business model, whereby some customers are recruited as agents or partners and incentivised to sign up others. Even excluding these ‘agents’ the group gets one of the highest scores. Eleven companies achieved customer scores of 70% or higher.

Npower was bottom of the table, with Spark Energy and Extra Energy next. Although Npower consistently sits at the bottom of the table, it is bottom by a smaller margin this year. Spark Energy in particular saw a marked deterioration, holding the joint-largest percentage of very dissatisfied customers (5%) with Npower. It dropped from 12^th place last year.

Of the Big Six, EDF Energy and EON were rated highest, scoring average markets. Scottish Power and British Gas are in the bottom five with Npower. All raised the prices of their standard variable tariffs over the last year.

The survey comes at a time when energy providers are under increasing scrutiny. Energy regulator, Ofgem, recently found that the large energy companies are overcharging 13 million customers by as much as £360 a year by letting them languish on high Standard Variable Tariffs (SVTs). Which? said those on expensive SVTs could save £333 a year by moving to the cheapest deal, which tend to be concentrated in the smaller firms.