Scottish Power said it was forced to raise bills because it "can no longer continue to shield our customers" from cost increases.
Neil Clitheroe, Scottish Power's chief executive for Energy Retail & Generation, said the company's costs have risen across the board for buying gas and electricity, supplying it to homes and dealing with customers.
"I find it very difficult having to increase our prices," Clitheroe said. "Each of these costs have been rising over the course of this year, and we have now reached a point where we can no longer continue to shield our customers from these increases."
Scottish Power has increased its gas and electricity bills as rifts appear in the Coalition over green energy taxes.
David Cameron has announced a review of competition in the energy market and vowed to roll back green regulations.