Prices will rise by an average 12.2% for dual fuel customers (who have gas and electricity) from 1 October in a bitter blow to 3.3 million households.
- Npower prices up average 12.2% for dual fuel
- 5th of big 6 firms to announce hike
- Average bill up £135 a year
Npower says a typical dual fuel customer on a standard tariff who pays by direct debit will pay £135 more per year.
Anyone with gas only will see an average 15.7% rise, while electricity-only customers will pay 7.2% more.
The rise affects all Npower customers other than those on a fixed or capped tariff, or those on its Sign Online 22 and 23 deals.
Scottish Power increased prices on 1 August, while British Gas costs are set to soar on Thursday.
Eon prices will rise on 13 September, while Scottish and Southern Energy's (SSE) are due to rise a day later.
Npower, like its rivals, blames world events forcing up wholesale prices (what suppliers pay for energy) as the reason for upping consumer prices this autumn.
Npower customers who'd benefit from switching can escape fee-free from variable tariffs that appear to lock them in.
Many gas and electricity deals come with an exit fee of up to £100 (though typically around £60) for those who leave before the term of the deal ends.
But where a supplier puts prices up, because that results in a contract change that's detrimental, rules from regulator Ofgem mean many consumers don't need to pay that penalty as long as they inform their power firm they wish to switch before the increase.
Npower confirms if you contact it before the price hike on 1 October to say you wish to leave, you'll escape a penalty.
The amnesty does not apply to those on a fixed rate deal because the price of their tariff will not rise, due to the nature of the product.
The same principle applies for British Gas, Eon and SSE customers who can also escape from variable tariffs before the rises come into effect.
However, it is now too late for Scottish Power customers.