More than 1.7 million Npower customers could save £261 a year, simply by moving to another Npower tariff.
As many as 6.6 million British Gas customers could save £129 a year, according to Ofgem.
However British Gas says it has changed its prices, so the saving is now £43.
The figures also reveal that SSE has almost all of its customers on the most expensive form of tariff.
91% of SSE customers are on standard variable tariffs, compared to just 9% at First Utility.
The number of people stuck on standard tariffs is controversial, as most fixed-rate deals are cheaper.
In the UK as a whole, 66% of households are on standard tariffs.
However Npower said that customers were unlikely to be able to save £261, as fixed-price deals were usually limited to a certain number of customers.
Once a certain number of people have signed up, the deal is withdrawn.
Last month, the business and energy secretary, Greg Clark, met energy suppliers, amid claims that some of them were making large profits from customers who are on standard variable tariffs.
The industry denied profiteering, but has nevertheless come under pressure to reduce its prices for standard tariffs.
Three of the biggest suppliers have frozen their tariffs through the winter as a result.
Nevertheless Ofgem published its comparison table to highlight the difference between the cheapest ands most expensive tariffs.
"Millions of people across Britain continue to pay too much for their energy," said Mr Clark.
"The measures announced today are a positive step to help more people benefit from increased choice and competition."
Citizens Advice said the new table would help customers, but that energy firms could still do more to help.
"We'd like to see energy companies do more off their own back to help customers who are on a low income, like moving them to a cheaper deal," said Gillian Guy, the chief executive of Citizens Advice.
Meanwhile the Competition and Markets Authority (CMA) confirmed that is going ahead with plans to help the 10 million households which have been on standard tariffs for more than three years.
From next year, suppliers will have to tell Ofgem which of its customers have not switched tariffs.
If customers consent, they will be sent details of better deals, prompting them to move.
Millions of energy customers could save hundreds of pounds a year on their energy bills without even switching supplier, according to the regulator Ofgem.