Consumers who switch to the deal will pay the same price for energy until the end of February 2016 - with an average yearly energy cost of Â£1,301.
The deal, rolled out on Thursday April 25, trumps a fixed price deal launched by npower just a day previously.
npower's deal offers fixed prices until December 2015, and costs, on average, Â£1,305 per year.
The average costs are calculated based on an energy user consuming 16,500 kWh of gas, and 3,300 kWh of electricity, with early exit fees accounted for.
The figures are also based on the assumption that a customer pays in arrears, by Direct Debit.
Although ScottishPower's deal offers the longest fixed-price period, there are cheaper tariffs available.
EDF offers its Energy Blue + Price Promise deal which, based on the same criteria, costs Â£109 less than ScottishPower's deal - and customers aren't charged exit fees. However, prices are fixed for a year less than ScottishPower's new tariff.
SSE Discount Energy Bonus takes the crown as the cheapest variable price tariff, with average costs per year of Â£1,146.
All of the Big Six are participating in the Government's Green Deal scheme, which offers customers efficiency improving measures for their homes, such as new boilers.
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