A new report by the Library of Congress on residential energy tax credits finds "little if any" evidence that the credits have proven to be an effective energy conservation tool.

According to a statistical analysis being prepared by the Library's Congressional Research Service, the price of energy is by far the most significant factor in encouraging conservation--about 10 times more important than tax credits.

The second most significant factor, a drop in household income, is seven or eight times more important than tax credits in prompting residents to cut home energy use.

CRS tax analyst Salvatore Lazzari, who is preparing the study, said the report will be completed in a few months. Until then, his conclusions are tentative, he said, but he does not expect the key findings to change.

The report is likely to be examined closely by the administration, which has indicated it would like to repeal the credits, and by the renewable energy industry, builders' groups and others that strongly support the tax measure.

A suggestion by President Reagan last fall that he may seek their repeal prompted a majority of members of Congress to cosponsor resolutions opposing the move.

Energy tax credits have been in effect since April 20, 1977, and are scheduled to expire Dec. 31, 1985. Individuals may claim a credit of 15 percent of the first $2,000 invested in home conservation improvements, such as insulation or storm windows and doors.

A 40 percent credit is given on the first $10,000 spent for renewable energy equipment, such as a solar hot-water system or solar heating equipment. Thus, a taxpayer who takes full advantage of the credits could deduct up to $4,300 on his federal tax return. In addition, according to Carol Kobrovsky, of the Conservation and Renewable Energy Inquiry and Referral Service, 28 states offer residential energy tax credits. Maryland, Virginia and the District are not among them.

According to the Internal Revenue Service, 10.8 million tax returns in 1978 and 1979 claimed energy tax credits totaling $1.1 billion.

Lazzari said his study has "data limitations" since the tax information is available only through 1979. "I doubt that additional years' data would change the conclusion, however," he said. "The other variables, particularly price, were just too important."

In his findings, he said, "Price was always important. Income was always important. Tax credits were virtually insignificant."

Although he does not specifically comment in his report on the possible effect that repeal would have on conservation, Lazzari acknowledged that the analysis suggests it would have litle impact. "As long as energy prices increase, the analysis indicates that the trend to keep consumption down will continue," he said.

The trend has been for energy consumption to drop about 5 percent in the short run as prices rise 10 percent, he said. As household income is cut by 10 percent, energy use decreases about 3 percent. Over a longer period, the decrease in energy use becomes greater, he said.

The analysis also indicates that tax credits seem to favor middle and upper-middle income households, he said.

Families with incomes under $10,000 claimed only 5.4 percent of the credits in 1979, while those with incomes over $20,000 claimed 70 percent.

Congress established the tax credits in the Crude Oil Windfall Profit Tax Act of 1980, in an attempt to encourage a reduction in the household use of nonrenewable fossil fuels. Jerry Kline is editor of Energy and Housing Report, published here by Energy Papers Inc.