HOUSTON - Coal stockpiles at U.S. power plants this week fell 2.3 percent from last week as much colder temperatures covered much of the nation, increasing power demand for heat, Genscape said Tuesday.

Total coal inventory dropped to 8.7 percent greater than the same week of 2008, shrinking from more than 12 percent seen a week ago, as previously idled coal-fired plants went to work.

Power generators had 181.4 million tons of coal on hand as of Tuesday, down from 185.7 million tons last Tuesday and compared with 166.9 million tons the same week last year, the power industry data provider said.

As of Tuesday, U.S. generators, which rely on coal to fuel about half of U.S. electricity production, had an average of 70 days' supply of coal on hand assuming typical burn rates, Genscape said, down from 71 days.

Power companies as of Tuesday averaged six more days' of coal stockpiled than the same week last year, down two days from 2008 stockpiles, data showed.

Many coal-fired plants are back online after being idled in the fall due to the slow economy that reduced power demand and increased use of cheaper natural gas-fired plants, Genscape said.

Coal inventories typically grow in spring and autumn when demand for heating and cooling drops. Stockpiles shrink during summer and winter when demand rises for climate control in homes, stores and factories.

The seasonal coal stockpile build has been slow this year since the inventory was unusually high at the end of the summer. Coal producers have responded by slowing shipments, with partial success.

Mathematical rounding sometimes affects the results, overstating some changes and understating others, Genscape has said. (Reporting by Eileen O'Grady; Editing by Lisa Shumaker)