By Karolin Schaps and Susanna Twidale
LONDON (Reuters) - Nearly all customers of Britain's second-biggest energy supplier, SSE (LON:SSE), are paying standard tariffs rather than cheaper rates on offer, a league table published by regulator Ofgem showed on Wednesday.
The league table, which for the first time records how many customers pay more expensive tariffs, showed that 91 percent of SSE's customers are paying a standard tariff. That is by far the largest percentage among the country's Big Six suppliers, with Centrica's British Gas in second spot at 74 percent.
So-called standard variable tariffs (SVTs) are basic rates that energy suppliers charge if a customer does not choose a specific tariff, a mechanism put in place to ensure consumers are always supplied with energy.
On average, the Big Six's SVTs are 132 pounds more expensive than their cheapest tariffs, the league table showed.
SSE's nearly 4 million customers placed on an SVT are paying nearly 100 pounds more a year than those customers placed on its cheapest offer, according to the table.
"We regularly tell customers about the cheapest tariff available to them so they can make informed decisions," said an SSE spokesman, adding the company's retail profit margin is around 5 percent.
Energy bills have doubled in Britain over the past decade to an average of around 1,200 pounds a year for gas and electricity combined, and the government said it could intervene in the market if it believes prices are too high.
Business and Energy Secretary Greg Clark, who has previously said he wants energy companies to treat all customers fairly, welcomed Ofgem's move to publish how many customers pay more expensive tariffs.
The league table also showed that Innogy-owned Npower has the highest difference among the biggest suppliers between its SVT and cheapest tariff at 261 pounds.
Other Big Six energy suppliers are Iberdrola's Scottish Power, E.ON and EDF (PA:EDF) Energy.
The table reflects the fact that over the past two years millions of customers have moved away from big providers to smaller companies.
The percentage of SVT customers at some of the small suppliers is as low as 9 percent in the case of First Utility.
Analysts at Jefferies said that further switching away from expensive tariffs would hit Centrica (LON:CNA) hardest.
"Centrica will be most exposed to customers switching to cheaper fixed tariffs," they said. Centrica, as Britain's biggest energy supplier, has 6.6 million customers on SVTs.
However, latest switching data published on Wednesday by industry group EnergyUK showed that more and more customers are returning to or staying with big suppliers.
A record 92,632 customers switched from a small to a large supplier in October and the figure remained high in November at 74,924.