Some 3.3 million Npower customers are facing higher energy bills after the company became the latest utility provider to hike its prices.
Average tariffs for gas will increase by 15.7% and electricity by 7.2% from October 1, Npower said, meaning the average monthly direct debit dual-fuel customer will see bills increase by 12.2% - an additional £134 a year.
The fourth largest provider in the UK blamed rising wholesale prices on the global market and a commitment to invest in future energy supplies.
The price hike comes at a time when household budgets are being stretched by the high cost of living and muted wage growth. Figures confirmed inflation in July increased to 4.4% from 4.2% the previous month.
Npower is the fifth of the "big six" providers to announce a price hike, following Scottish and Southern Energy, Scottish Power, British Gas and E.ON. EDF is yet to announce changes to its tariffs.
The price hike is likely to provoke further outrage as it follows a stellar set of half-year results from Npower - which more than doubled its profits in the six months to June 30.
RWE, the parent owner of Npower, said its British operations made operating profits of 352 million euro (£309 million) in the period, up from 154 million euro (£135.2 million).
However Npower said its price rises were lower than its competitors and the company also introduced a new deal which fixes the new rates until March 2013, protecting customers who sign up for the next two winters.
Adam Scorer, director of external affairs at independent lobby group Consumer Focus, said Npower's customers will have been "dreading this announcement".
He said: "It will not come as a surprise, but it will be a source of great frustration when profits are up."