HOUSTON, June 23 (Reuters) - Coal supplies at U.S. power plants fell 0.03 percent this week from last week but are 20.2 percent greater than the same week of 2008, Genscape said Tuesday.

The week-to-week decline of 55,000 short tons was the first since February and reflected output falling into balance with reduced demand during the recession, the industry data provider said.

Electric companies had 178.9 million tons of coal stockpiled, compared with 179 million tons reported last Tuesday and 148.8 million tons the same week last year.

U.S. generators as of Tuesday had an average of 66 days' supply of coal, assuming typical burn rates. That equals last week's coal-burn capacity, the industry data provider said.

As of Tuesday, power plants had 11 more days' supply than the same week last year. That is equal to last week's margin over 2008 stockpiles, Genscape said.

As miners cut output, U.S. coal production through June 13 has fallen more than 30 million tons behind the same period last year, the U.S. Energy Information Agency has said.

The Mid-Atlantic and Midwest still saw stockpile growth, largely due to mild weather keeping a lid on power demand, Genscape data show.

The Plains, Texas and Southeast led stockpile reduction. Texas weather has been hot for a while, but coal supply kept growing as utilities used cheap gas to generate power, Genscape said.

U.S. coal stockpiles usually grow in the spring and fall, when mild weather eases cooling or heating demand. Stockpiles shrink as summer or winter sets in across the country, boosting demand for electricity for cooling or heating.

Mathematical rounding sometimes affects the results, overstating some changes and understating others, Genscape has said. (Reporting by Bruce Nichols; Editing by David Gregorio)