This winter's wholesale natural gas price spikes and volatility have sent a shiver into the projected second quarter energy utility bills for Colorado homeowners and small businesses getting their gas and power supplies from Xcel Energy Inc.

The Minneapolis-based utility that operates as Denver-based Public Service Company of Colorado said Monday it will file with state regulators to raise gas and electric utility rates, with the gas charges increasing residential bills an average of 15% and small business bills 19% next quarter, compared to bills for the same period last year. Electric rates will go up less than 5%.

Increases for both gas and power are attributable to what an Xcel spokesperson called "higher than anticipated gas prices resulting from colder than expected weather" in the first quarter. The higher wholesale prices are reflected in Xcel's gas cost adjustment and electric commodity adjustment.

If the Colorado Public Utilities Commission approves the request, the new rates will be effective April 1.

For residential gas utility customers, the commodity price has gone from 42.84 cents/th in the second quarter last year to 53.92 cents/th in the second quarter this year, driving monthly bills from a little more than $30 to more than $35. For small business gas customers, the commodity rate went from 42.5 cents/th to 53.59 cents/th, and monthly bills from $128 to nearly $153, quarter over quarter.

For electric utility customers, the monthly bills will go up by 4.4% for residential and 4.8% for small business customers compared with power rates in the first quarter this year.