The move comes amid widespread expectation that energy prices will rise later this year. Prices from the ‘Big 6’ suppliers rose by 6 – 11% over the autumn and winter 2012/13.

The new deal is not the cheapest fixed tariff in the market. At an average annual cost of £1,305.39 it is more than the £1,181.94 Blue+ Price Promise June 2014 from EDF Energy and the £1,171.68 12-month fix from OVO Energy, although this deal is not available in the Scottish Hydro region.

The appeal of the npower deal is the length of the fixed price guarantee, which serves as a 30-month shelter against price hikes during the period.

Along with other fixed tariffs, it is up to £60 per annum cheaper than the average price of a standard tariff from the Big 6 suppliers, although Cooperative offers a standard tariff at an average of £1,157. These tariffs have no insulation from price rises.

The npower offer gives customers the option to pay their bills by Direct Debit or on receipt of the bill, with those paying for both fuels by Direct Debit eligible for a discount of up to £100.

Price Fix December 2015 is not available to customers with pre-payment or multi-rate meters.

Last week npower announced that, from May 1, 2013 its standard tariffs will have a single rate for each unit of energy used, which means seasonal weightings will be removed. The firm will also levy a fixed standing charge “to cover the cost of bringing gas and electricity to the (customer’s) door.”

The annual standing charge for Price Fix December 2015 will, for average customers, be £24 higher than for the standard tariff.

Those opting for the new fixed rate deal who want to change tariff or supplier before the end of the term will be hit with a £50 per fuel exit fee. If they stay to the end of the fixed period, they will subsequently be moved over to npower’s standard offline variable tariff.

At this point – or preferably a few weeks before to allow for switching – customers should search the market to find the most competitive fixed rate deal.

If you’re tempted by the potential savings on offer through switching, visit out energy switching page for details of offers from npower and the other leading energy providers.

Follow Kevin on Twitter @KevinPrattMSM

Please note: Any rates or deals mentioned in this article were available at the time of writing. Click on a highlighted product and apply direct.