Defying the giant American Hospital Association, a group of 100 community hospitals will announce support today for President Carter's strict hospital cost guidelines.

At a White House news conference to be held this morning, anti-inflation chief Alfred E. Kahn and Health, Education and Welfare Undersecretary Hale Champion will accept the National Council of Community Hospitals offer to try to hold 1979 expenses to a 9.7 percent increase.

President Carter called for that goal for the nation's hospitals in January. American Hospital Association President Alex McMahon promptly called the figure "impossible" to meet in view of the high costs of the fuel and materials hospitals buy, as well as other expenses.

But McMahon promised that hospitals will meet their voluntary goal of increasing their total spending by no more than 11.6 percent, if general inflation remains within predicted ranges.

Yesterday John Horty, president of the community hospitals council, wrote to the president that its members will try to meet his tight goal.

"We're doing it because we believe inflation is the nation's number one domestic problem, and hospitals ought to do all they can to fight it," Horty said.

The association's 100 members represent a fraction of the country's 7,000 hospitals, but possess more than 5 percent of their total beds. Members are all nonprofit, nongovernment hospitals, and include Greater Southeast Community Hospital in Washington, and such well-known institutions as Mercy Hospital in Miami, Holy Cross Hospital in Salt Lake City, the Scripps Hospital and Clinic in San Diego and the Ochsner Clinic and Hospital in New Orleans.

The American Hospital Association, the investor-owned Federation of American Hospitals and the American Medical Association early last year launched their own "voluntary effort" to limit hospital spending.

Last week the AHA reported that in the first 11 months of 1978, hospital expenditures increased by 12.9 percent, down from a 15.8 percent increase for January-November 1977.

Administration officials said in January that their new anti-inflation crusade should help the hospitals do better. "But the administration based its hospital goal on a mere 7.9 percent increase in the hospital marketbasket, the things hospitals must buy. The Congressional Budget Office just estimated that that marketbasket will go up 9.1 percent," said an AHA spokesman.

In the next two weeks White House officials expect to send Congress their latest hospital cost control bill to put a mandatory lid on hospital spending if hospitals fail to meet the administration goal.

A similar effort collapsed in the House Commerce Committee last year, although it was passed by the Senate.