The news comes after three of the UK's six main energy suppliers announced similar cuts in response to the falling wholesale price of gas.
Npower customers can now expect to see their annual bill reduced by an average of £50.
Discounts for those who pay by monthly direct debit will also be revised, with discounts for electricity-only customers doubling to £40, and gas-only customers rising from £20 to £50. Dual-fuel consumers who pay by direct debit will continue to receive a £100 discount after 12 months.
Kevin Miles, Chief Executive of Npower Retail, said: "We always aim to offer competitive prices to our customers and we have lowered our gas prices although our profits halved in 2009."
Audrey Gallacher, from watchdog Consumer Focus, commented that, "The days of cut-throat price competition seem a distant memory. All suppliers now cut by almost identical margins within days of each other."
EDF Energy and Scottish Power are now the only "big six" suppliers yet to announce price cuts.