Xcel Energy announced that customers' natural gas bills are expected to rise significantly in December from November with the onset of winter and higher demand.

But the utility added that the gas bills households and small business customers pay next month would be "relatively flat" versus what they paid last December.

Xcel asked the Colorado Public Utilities Commission for permission to raise gas prices for residential and small-business customers by 16 percent next month from November levels.

Typical residential customers are expected to increase their gas use by 37 percent in December versus this month. Overall, residential bills would increase to $113.97 from $78.47 this month. Typical November 2007 bills were $111.04.

Typical small-business customers are expected to use 43 percent more gas. Their bills would increase to $532.97 from $344.10. Typical November 2007 bills were $518.72.

Xcel buys natural gas from the wholesale market and uses it to generate electricity and to sell directly to customers to heat homes. It does not profit on the commodity.