Video report by ITV Consumer Editor Chris Choi.
Npower has announced one of the biggest energy price rises ever implemented - with thousands of consumers set to see annual bills rise by £109 on average.
The company said it will raise standard tariff electricity prices by 15%, and gas prices by 4.8%, from 16 March.
This means a typical annual dual fuel bill will rise by an average of 8.9%, or £109.
Npower said the changes will affect about half of its 2.8 million customers, as the other half are on fixed-term energy deals, and will see no price increase.
It blamed increases on wholesale energy costs, and the costs associated with delivering government policies like smart meters
But Energy regulator Ofgem has said that it does not see "any case" for significant price increases where suppliers have bought energy in advance, and asked for the firm to justify its decision.
An Ofgem spokesman said: "Our new supplier cost index shows that costs for energy suppliers have risen over the past year after having fallen for the previous two to three years.
"However, we don't see any case for significant price increases where suppliers have bought energy well in advance. Npower must therefore justify the decision to its customers."
And a spokesman for Theresa May spokesman said the Government was "concerned" about the move.
The company is the second of the 'big six' energy firms to announce a price change.
In December, EDF Energy announced a reduction of 5.2% in gas prices, but a rise of 8.4% for electricity, which will come into force in March.
Three other 'big six' suppliers - British Gas, E.on and SSE - said that that would keep prices at the same rate until the end of March.
ITV News has asked Scottish Power if they are planning to make price changes this year.
- Watch 'Energy Bills: Can We Be Smarter? - Tonight' on the ITV Hub