Npower has scored the lowest in an annual consumer satisfaction report for the seventh year running.
Ovo Energy topped the list, which also saw the 'Big Six' continue to languish in the bottom half of the table.
Consumer watchdog Which? surveyed 9,000 people on their energy suppliers and questioned them on key measures including value for money, clarity of bills, handling of complaints and whether they feel the company helped them save money.
Npower achieved a customer score of just 44%, but E.ON (57%), British Gas and SSE (56%) as well as EDF Energy (55%) and Scottish Power (50%) all fell below many smaller suppliers.
Electric Ireland (80%) topped the list for energy suppliers in Northern Ireland, followed by Budget Energy (76%).
However no supplier scored highly enough to be awarded the Which? recommended provider status.
The full list is as follows:
- Electric Ireland (NI only) - 80%
- Ovo - 78%
- Budget Energy (NI only) - 76%
- Places for People Energy - 76%
- Ebico - 75%
- Flow Energy - 73%
- Good Energy - 66%
- E.ON - 57%
- British Gas - 56%
- SSE - 56%
- EDF Energy - 55%
- M&S Energy - 55%
- Scottish Power - 50%
- Npower - 44%
The consumer watchdog found that customers with the so-called 'Big Six' could save an average of £330 a year by moving to the cheapest dual fuel deal on the market this winter.
Which? Magazine editor Richard Headland said: "Yet again we have found the Big Six energy firms performing poorly and smaller suppliers leading the way when it comes to customer satisfaction.
"Larger suppliers need to urgently up their game in line with smaller providers.
"Energy companies should now be doing much more to genuinely engage their customers who are stuck on these poor-value deals.
"If nothing is done, then the energy companies will have no-one to blame if the Government and the regulator have to step in to ensure this market starts working for consumers."