The supplier will increase its electricity prices by 9.3 per cent and its gas prices by 11.1 per cent from 1 December.
The company claimed the main reasons behind the price rise are increases to distribution charges, the cost of meeting government obligations, and the increasing wholesale prices.
Paul Massara, chief executive at Npower, said: “I know that increases to household bills are always unwelcome, and this is not a decision that we have taken lightly.
“We will continue to take steps where we can to reduce the impact of the external influences on energy bills.
“We only aim to make around 5 pence in every pound (5 per cent) in our retail business which we feel is a fair return for delivering reliable energy to consumers and for the risks that we bear.”
Massara also attacked Labour’s proposed price freeze, saying it would not lead to lower prices “because it doesn’t cut the growing costs of supplying energy”.
He added that “imposing price controls discourages investment, increases uncertainty and ultimately leads to higher prices”.
Npower’s price rise follows similar rises announced by British Gas and SSE, while Co-operative Energy also plans to increase it’s electricity and gas prices by an average of 4.5 per cent.
Alongside the price increase, Npower also announced that it will be simplifying its gas standing charges by removing regional variability and introducing a single national equivalent.
The new gas standing charge will reduce from an average of £146 to £100, although this reduction will be balanced by an increase in unit rates.