The Senate Finance Committee has so battered President Carter's proposed windfall profits tax that in its present form it would recapture less than a third of the $226.6 billion in extra revenue the oil industry is expected to reap from price decontrol.

Estimates show that the bill the panel is drafting would have an effective tax rate of 29 percent on the "windfall" that the industry would receive over the next 10 years after payment of other taxes.

By contrast, the windfall bill passed by the House, which closely paralleled the president's own proposal, would impose an effective tax rate of 43 percent on the windfall -- almost half again as much as the Finance Committee plan.

Moreover, Finance Committee Chairman Russell B. Long (D-La.) has said the panel will give away at least half the new revenues from its tax in the form of tax credits to spur conservation, leaving only $35 billion for the other energy uses Carter prefers.

Carter had asked the lawmakers to devote all the proceeds from the new tax to his proposed synthetic fuels development plan and other programs. The House bill would earmark more than $100 billion for this.

The president's decision to lift price controls on oil is expected to increase oil companies' revenues by as much as $400 billion over the next 10 years, as domestic oil prices rise to world levels.

Of this $400 billion, the companies are expected to pay about $173 billion in federal income taxes over the period, reducing the so-called "windfall" for the decade to about $226.6 billion.

The windfall tax approved by the House would take $104 billion of this, offset by $7 billion in extra deductions on state taxes. (The windfall tax is an excise tax, and so is deductible.)

That puts the measure's total tax take for the 10-year period at $97 billion, or 43 percent of the $226.6 billion. The oil companies would be left with $129.6 billion.

The Finance Committee's version would trim the 10-year take from the windfall measure to $70.5 billion. But the net, after the offsetting new deductions on state taxes, would be $65.5 billion over the 10 years, for an effective tax rate of only 29 percent.

Tax experts are quick to point out that some of the Finance Committee tax-paring could lead to increased production -- for example, its decisions to exempt revenue from newly discovered oil and high cost tertiary oil from the windfall tax.

However, the panel also exempted stripper oil, which some energy experts see as unnecessary. And the committee may act this week to exempt Alaskan oil from the windfall tax, at a potentially high cost.

The Finance Committee begins work today on a plan by Long to limit the cutbacks to the current level and put a lid on the tax credits that panel members want to tack onto the bill.

The panel earlier had voted tentatively to approve about $99 billion in energy tax credits, but Long later announced plans to hold that to about $35 billion, for fear the committee's proposed tax breaks would outstrip the tax itself.

Carter originally requested a windfall tax that would raise $110.7 billion over the 1980-1990 period, calculated at today's world oil prices of $22 a barrel. After the House passed its $104 billion bill, Carter raised his request to $118.8 billion.

The windfall tax the president originally proposed was regarded as so mild that some analysts complained it was primarily a political gesture to offset the impact of Carter's earlier decision to lift price controls.

Even in the president's version of the tax plan, the windfall tax was overshadowed by the amount the oil companies would have to pay in federal income taxes on their increased revenues. Decontrol began last summer and is to take effect gradually, with controls disappearing entirely in October 1981.