The consumer group surveyed 8,000 people across the UK about their provider covering a range of factors, including customer service, value for money and accurate billing.

Five small suppliers topped the rankings: Octopus Energy claimed first position, while Robin Hood Energy and So Energy were in joint second place. They were followed by Ebico and Tonik Energy in joint fourth position.

The Big Six energy suppliers – British Gas, EDF Energy, Eon, Npower, Scottish Power and SSE – were all in the bottom third of the table.

Alex Neill, Which? managing director of home products and services, said: “Our survey shows the importance of value for money and good customer service – energy suppliers delivering both to their customers tend to be thriving, while the Big Six and other poorly-ranked firms are paying the price for not giving customers what they want.

“All energy companies should be working to give their customers competitive prices and excellent customer service. If you’re not getting a good deal, you should look to switch, as you could potentially save over a hundred pounds and get a much improved service.”

Customer service

Octopus Energy, which supplies more than 400,000 homes, was rated as good or excellent value for money by nine in 10 customers and 96% of its customers gave it the same rating for online customer service.

Small supplier, Solarplicity, rated for the first time in the Which? survey, was the worst energy firm according to its customers, with an overall score of 44% compared with Octopus’ 80%.

SSE was the highest-ranked among the Big Six – and the only firm to receive four star ratings for its customer service online and on the phone.

Meanwhile, Npower and Scottish Power were the lowest ranked among the Big Six, receiving only two two stars for their customer service on the phone.

Which? found less than one in five (18%) Big Six customers were very satisfied with their supplier, compared with a third (33%) of customers with medium and small suppliers.

Three small firms included in the Which? survey – Spark Energy, Extra Energy and Economy Energy – have ceased trading since the survey was conducted.

Spark was taken over by Ovo Energy, which has retained the Spark Energy brand. Economy Energy and Extra Energy are no longer trading so aren’t included in the Which? results.