Commerce Secretary Juanita M. Kreps said yesterday the $15 billion tax cut for individuals the administration has been planning to help stimulate the economy would merely enable consumers to keep pace with the impact of inflation and higher Social Security payroll taxes.

In a speech before the National Press Club. Krebs said a cut of at least $15 billion will be required simply "to prevent taxes from rising as a per cent of presonal income" and to "reduce the drag on consumer spending" that results from those developments.

The Secretary's estimates were challenged by some economists, both in and outside the administration and were not taken as official policy. Other analysts estimated it would take about $11 billion to $12 billion in tax cuts to offset inflation and higher payroll taxes.

Nevertheless her statement marked the first time the administration has admitted that most, if not all, of its planned tax cut for individuals would simply permit consumers to stay even with inflation and would not provide substantial new purchasing power.

Treasury Secretary W. Michael Blumenthal told a nationwide telecision audience Sunday the administraion is considering tax cuts of $15 billion to $10 billion, with about three-fourths of that earmarked for individuals.

Carter told a press conference Oct. 13 that his coming tax cut proposals would be aimed in part at stimulating the economy. He said his decision on whether to seek tax reductions in 1978, rather than 1979, would depend largely on the state of the economy. Kreps suggested yesterday that Carter split the tax package into two stages - proposing a general tax cut and some modest changes next year, and dealing with more controversial tax recision in 1979. She also urged him to propose a sizeble business tax cut.

The Secretary predicted that, without a substantial tax cut, the economy's performance in late 1978 and beyond is "likely to be below the pace needed to achieve our goals."

She estimated that the growth rate in the final three months of this year wold be above the 3.8 per cent pace recorded last quarter.

Kreps' calculation that a $15 billion tax cut is needed to prevent erosion of consumer purchasing power was hased in large part on her estimate of the impact of inflation on the income tax system. As incomes rise, spurred by inflation, taxpayers are pushed into higher brackets and individuals' tax burdens increase.

Commerce Department economists said the Secretary estimates it would take 19 billion or more in tax cuts to offset this phenomenon, and another 15 billion or so to compensate for the higher Social Security taxes now being voted by Congress. Kreps said the cuts should take effect in 1978.

Meanwhile, Blumenthal criticized the Senate version of the energy bill yesterday as a virtual raid on the Treasury. He predicted it would cost taxpayers at least $55 billion by 1985, and would "conserve far less energy at a much higher cost" than the House bill.

The Secretary made his remarks in a speech before the Indiana Chamber of Commerce in Indianapolis. A text of the address was made available by his office here.

In her speech yesterday, Kreps urged the administration to develop special programs to provide more jobs in big cities and to "cushion the shock" of those who are thrown out of work because of competition from foreign imports, such as steel.

She also conceded that the administration's new trade adjustment assistance program, now on the President's desk for final action, probably would have limited impact because so little money would be involved. She said the government had to stimulate more new jobs to help the unemployed.