The price rise will take effect on 1 December, and is the highest increase announced by any supplier so far.
SSE will increase prices by 8.2% from 15 November and British Gas said prices would go up by 9.2% on 23 November.
The Npower increase includes an electricity price rise of 9.3% and a gas price rise of 11.1%. The move will affect 3.1 million customers.
The change will add an extra £137 to an annual average dual-fuel bill, taking it to £1,459.