Millions of families are in danger of being burnt by an extra £300 as fixed rate energy deals come to an end.
Households are being warned to switch when the money saving tariffs expire – or face a hike when they are put back on a standard charge for gas and electricity.
A study by website MoneySupermarket.com found customers on Npower’s Go Fix II will have to find an extra £319 when their contract ends in three weeks time.
Bill payers who fork out an average £1,033.19 will face an annual charge of £1,352.23 but could save £160.58 if they switched to another tariff.
And the energy giant’s customers on its Bill Saver May 2012 deal will be £265 out of pocket when the contract ends on May 21 with bills going from £1,086.39 to £1,352.23 overnight.
According to the research, Scottish Power customers on the Fixed Saver June 2013 tariff will be down by £258 when the deal finishes at the end of the month as bills rise from an average £1,109.71 to £1,368.45.