The Big Six energy companies have all ranked in the bottom 10 of an annual customer satisfaction survey as consumers turned to medium and small suppliers.
However, some small suppliers are showing signs of "growing pains", with Extra Energy and Spark Energy joining Npower at the bottom of the Which? customer satisfaction survey.
Utility Warehouse, Flow Energy, Octopus Energy, PFP Energy and Utilita took the top five spots based on a poll of almost 9,0000 people.
Scottish Power and British Gas took joint 26th position, below SSE at 24 and EDF and E.On in joint 22nd position.
The survey found only a third (32 per cent) of customers with the Big Six are very satisfied on average, compared with 52 per cent for medium-sized suppliers and 45 per cent for smaller suppliers.
Customers of medium-sized energy firms are also most likely to rank their supplier as excellent for value for money, the clarity and accuracy of bills and phone customer service.
Which? made just one energy company a "recommended provider" this year - the small supplier Octopus Energy.
It found customers with the Big Six on a standard variable tariff could save up to £333 a year by moving to the cheapest dual fuel deal on the market.
Alex Neill, Which? managing director of home products and services, said: "Energy customers shouldn't tolerate shoddy service, sky-high prices or failure to resolve complaints.
"Once again our survey shows many of the biggest energy companies languishing at the bottom of the table, with some small suppliers showing signs of growing pains.
"Energy companies should be working to give their customers a fair deal, which includes competitive prices and good customer service. If you're not getting a fair deal you should switch away today to potentially save hundreds of pounds and get better service."
Npower said: "It is disappointing to see these results as we're currently receiving the second lowest number of complaints per 100,000 customers out of all the Big Six energy suppliers and our own internal customer satisfaction also shows we're improving.
"Clearly, there's more for us to do and we'll continue working on improving our position in this survey."
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