The company said this week that 3.3 million customers will see price rises of 15.7 per cent for gas and 7.2 per cent for electricity from October.
The average annual increase will be around Â£133 for dual-fuel customers, whilst the rises themselves - similar to those introduced by British Gas, Scottish Power, Scottish and Southern and E.ON - are far above inflation.
Like the other energy suppliers, Npower has said rising wholesale costs are behind the price hikes.
The raft of price rises come amongst huge profit reports for each of the energy companies - leading MPs to question whether the rises were necessary.
Whilst the Big Six have stuck to blaming rising wholesale costs, recent figures show wholesale costs are actually dropping. Consumer Focus, a customer watchdog group, said Npower customers were "dreading" the rises after the first four suppliers announced their hikes.
"It will not come as a surprise, but it will be a source of great frustration when profits are up," said Adam Scorer, from the group.
He did point out that Npower's rises are actually lower than many of their competitors', but added: "With wholesale costs around a third lower than their peak, understandably people are questioning whether it is right that customers face all-time high prices."
Energy watchdog Ofgem is already investigating several companies over their sales techniques - with recent calls to ban all doorstep selling. Other investigations are ongoing into the apparent over-complication of various energy tariffs, too.
According to the Telegraph, MPs are calling for a 'competition commission' to be set up, to check if the Big Six have a 'stranglehold' on the market.
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