Norwich - Adjustments in electric and gas power purchases by Norwich Public Utilities will mean good news for city natural gas customers this heating season and bad news for electric customers in the coming months.

NPU officials announced adjustments Tuesday effective with December "consumption" and the January bills. Purchased power adjustments for electricity and purchased gas adjustment are changes to customers' bills based on the cost NPU incurs in buying wholesale power from the New England electric grid or from natural gas distributors.

Average residential customers will see an increase of about $19 per month in their electricity bills starting with their January bill. Residential gas customers will see a drop of about $37 per month. NPU officials said gas and electricity usage among commercial and industrial customers varies greatly.

Seventy-five to 80 percent of residential customers receive gas and electricity from the municipally owned utility.

Stephen Sinko, NPU division manager, and General Manager John Bilda said power adjustments differ from regular utility rates, which cover more predictable expenses such as operating costs, capital improvements and debt service. With purchase power and gas adjustments, utility officials study the very volatile wholesale markets and adjust billing accordingly.

Bilda said NPU lowered the electricity adjustment in August 2009 to help customers during peak cooling months. Utility officials timed the new gas adjustment reduction to coincide with peak heating months, Bilda said.

Both adjustments are expected to be "solid" for the next nine to 12 months, Sinko said. That doesn't mean the actual purchase prices for gas and electricity will be stable, however.

A line graph charting purchase power costs from July 2008 through projections for June 2011 is a jagged path of sharp increases and decreases, from just over 12 cents per kilowatt hour in July 2008 to under 8 cents by December that year and up again past 12 cents by January 2009. The rate reached a high of over 14 cents per kilowatt hour in March and April 2010, but NPU kept its lower rate through that spike.

A similar but longer-term graph for gas costs showed even more dramatic price swings from $10 per 100 cubic feet of gas in January 2001 down to under $2 a year later and back up to $14 by January 2006. The recent trend between January 2009 and projected through January 2012 puts the price range between about $2.50 and $6 per 100 cubic feet of gas.

Sinko said NPU doesn't try to adjust for actual month-to-month fluctuations but tries to provide customers some stability in their bills by making adjustments no more frequently than quarterly.

c.bessette@theday.com