Ofgem said the level of the price cap for default and standard variable tariffs (SVTs), which protects around 11 million customers, will fall by £75 to £1,179 from October 1 due to lower prices in wholesale energy markets.
The regulator added that the price cap for around 4 million pre-payment meter customers will also fall - by £25 to £1,217 a year from October 1.
It said wholesale energy costs had fallen “significantly” since February, when it last reviewed the price caps.
Ofgem chief executive Dermot Nolan said: “The price caps require suppliers to pass on any savings to customers when their cost to supply electricity and gas falls.
“This means the energy bills of around 15 million customers on default deals or pre-payment meters will fall this winter to reflect the reduction in cost of the wholesale energy.”
But Ofgem stressed households are still better off by switching away from default tariffs and SVTs to get the best deals in the market.
“Households can cut their bills further in time for winter, and we would encourage all customers to shop around to get themselves the best deal possible for their energy,” said Mr Nolan.