FINALLY, ITS PASSED. After 18 months of the most dramatic and tortuous legislative politics, President Carter's energy bill is back where it started from, on his desk. That is the great overshadowing accomplishment of the 25th Congress. The Senate finally wore down a few members' campaign of delay and finished work on it early yesterday. The House took it up at 2:40 a.m., discussed it for four hours, and voted just before dawn on the last day of the session. But the bill returns to Mr. Cater in very different form than he wrote it. Exactly what has been achieved in this long and wearing struggle?

The real effect of this bill will lie far beyond the endless statistical quarrels over how many barrels of oil it may save. The bill seals into law and national policy a conviction that has been spreading through this country ever since the Arab oil embargo five years ago. It is the conviction - contrary to old and deeply engrained American ways of thinking - that, regarding energy, more is not necessarily better, either. You can see why this legislation was fought to the last ditch by large and loud segments of the oil and gas industry, in an involuntary alliance with most of the old-line consumers' organizations.

In retrospect, it's clear that the bill was drafted too fast and tried to cover too much. But the central principle was the right one - that it takes higher prices to enforce the kind of serious conservation now essential to protect the American economy from further shortages and crises. To avoide windfall profits, the prices were to be raised by taxes. To avoid enriching the U.S. Treasury, those taxes were to be rebated directly to the public. The original bill contained frou separate systems of tax and rebate, all of them hideously complicated. Congress didn't like the idea. The first, the gasoline tax, sank without a sound in the House. The other three - on crude oil, on industrial use of gas and oil, and on inefficient cars - wasted slowly away in the Senate. The only survivor is a vestigial tax on automobiles with grossly poor fuel efficiency.

Congress chose to rely mainly on the instruments that it knows and likes best: regulation and tax subsidies.It voted to let the price of natural gas rise graduatelly toward complete decontrol in the middle or later 1980s. That would end the absurd practice of keeping gas, the cleanest of fuels, cheaper than it s competitor, oil. This reversal of the past two decades' regulatory practice was the greatest threat to final passage of the bill. But in the end Congress accepted the view that prices have to reflect scarity, not wishful thinking.

As for oil prices, it's worth noting that Congress has not voted to keep holding them down. It merely chose not to take the initiative, itself, in raising them. Mr. Carter has promised to raise oil prices to world levels within the next two years and, under previous legislation, he has broad power to it. Going to world level would mean a price increase of 12 cents a gallon for oil products, plus whatever OPEC chooses to add.

The benefits of this legislation will not be measured by any immediate arithmetical effects on oil imports. The achievement here is that the bill moves American policy closer to the realities of a world in which oil and gas are going to be scare. The bill advances the national commitment to conserve. For the first time, Congress has dealt with the country's three major fuels - oil, gas and coal - together, in the knowledge that the price and availability of each have impacts on the others. True, the bill is battered and dented and stripped not only of its hubcaps but also some of the parts that the designers originally considered essential. The country is in the process of changing its mind, and that is never a tidy or orderly process. The bill will be historically important, we suspect, not for any theoretical symmetry or intellectual beauty, but because it represents a turning point in the way that Americans think about fuel and energy.