Npower has announced one of the largest single price rises implemented by a “Big Six” supplier.
The company will raise standard tariff electricity prices by 15% from 16 March, and gas prices by 4.8%.
A typical dual fuel annual energy bill will rise by an average of 9.8%, or £109.
Npower said the changes would affect about half of its customers. The other half are on fixed-term deals and will see no price rise.
However, 1.4 million customers on existing standard tariffs will be offered a four-year fixed-price tariff with a 4.8% discount.
The announcement comes after three other suppliers – British Gas, E.on and SSE – announced they would keep prices on hold until the end of March.
EDF cut its gas prices by 5.2% last month, but will raise electricity charges by 8.4% from 1 March.
Npower said it was the first time it had raised prices for three years.
“This is a hugely difficult decision, and we’ve delayed the date this takes effect until after one of the coldest months of the year,” said Simon Stacey, Npower’s managing director of domestic markets.
It blamed increases in wholesale energy costs, and the cost of delivering government policies such as smart meters and the renewables obligation.
However, two weeks ago the regulator, Ofgem, told suppliers they should absorb the costs of wholesale price rises by buying energy in advance.