Proposed federal climate change legislation may clean the air, but it won’t come cheap for Texans, according to a study by the state’s main electric grid operator.

Texans’ monthly electric bills could increase by $27 or more by 2013 if Congress passes a bill that puts a price on carbon dioxide and other common power plant emissions, according to the study by the Electric Reliability Council of Texas.Monthly bills could be higher or lower depending on the price of natural gas — an important fuel for power plants that essentially sets the price in Texas — the cost of the CO2 emissions and whether power use decreases in reaction to higher prices.

A draft of the federal climate change law proposes setting a steadily decreasing cap on emissions from factories, power plants and other industrial sources and letting companies trade any excess emissions allowances. The price of those emissions allowances would most likely be passed on to consumers in the form of higher electric rates.

It also would create a system for creating extra allowances, called offsets, through other projects that reduce emissions.

The ERCOT report says CO2 allowances must be expensive enough to persuade companies to actually cut emissions rather than simply buying allowances that let them continue to pollute.

That threshold is between $40 and $60 per ton, according to the study.

If natural gas costs $7 per million British thermal units and CO2 $40 to $60 per ton, it would mean a $27 increase to the average customer’s electricity bill.

If natural gas is $10 per million BTUs, the monthly cost increase could be closer to $33.95.

But if the higher-cost power causes people to cut their use, the typical monthly bill increase would be about $17, the study says. 

ERCOT did the study using a model similar to one used by PJM Interconnection, grid operator for Illinois, Pennsylvania, New Jersey and other Eastern states.

The study has shortcomings, some observers say.ERCOT clearly has the expertise to estimate power costs, said Victor Flatt, an environmental law professor at the University of Houston Law Center, but the study doesn’t include the potential impact of the offsets market. Offsets are expected to be a less expensive way to help companies meet their emissions goals in the early days of the market.

“The effect of an offset allowance is likely to be large,” Flatt said. “They also project only in the short term. In the middle and longer term, the effects of new technology, particularly carbon capture and sequestration, should have an effect.

”The $40 to $60 per ton carbon estimate seems a bit high to Luke Metzger of Environment Texas, noting that the Environmental Protection Agency’s analysis of the bill predicted CO2 prices of less than $20.

And while the study takes into account lower energy use due to higher prices, it does not take into account all the planned energy efficiency components of the bill. The Union of Concerned Scientists says those parts of the bill could save the average U.S. household $900 annually.