Energy Secretary James R. Schlesinger yesterday offered to sell, for the first time in four years, the nation's future output of enriched uranium for nuclear power plants.

"We can reopen the order books," Schlesinger said at a press conference, "to fulfill President Carter's commitment to make the United States a reliable supplier of enriched uranium to the world."

This means that electric companies here and abroad can again turn to the federal government for uranium through the 30-year lifetime of their nuclear plants.

One reason they can do so is that many who signed orders before the books were closed are in the midst of cancelling their plans to build nuclear plants, thus freeing additional enriched uranium. Another reason is that the United States is expanding its capacity to produce enriched uranium.

In the four years since the enriched uranium order books were closed, many nations planning nuclear power plants turned to France, the Soviet Union and a triumvirate of Great Britain, the Netherlands and West Germany for their supplies. All three suppliers expanded their enrichment plants to meet the fresh demand.

The United States has been anxious to maintain its dominance of the world's enriched uranium market for more than selfish business reasons. If it supplies another nation with enriched uranium, it has the power to negotiate the terms under which plutonium for weapons can be extracted from the uranium after the fuel is spent.

The three plants that enrich uranium with the fissionable isotape U-233 so it can be used to generate electricity are at Oak Ridge, Tenn., Paducah, Ky., and Portsmouth, Ohio. The three have the capacity to provide enrichment for no more than 335 million kilowatts of electricity.

That limit had nearly been reached in mid-1974 when the then-chairman of the Atomic Energy Commission, Dixie Lee Ray, suspended the order books for uranium enrichment.

Since then, companies that had signed up for orders before the suspension have said they want to terminate their contracts. These companies account for about 23 million kilowatts of nuclear generating capacity.

At the same time, the United States has begun to expand capacity at the three existing plants and has authorized construction of a fourth plant at Portsmouth, Ohio. The fourth plant, using the new centrifuge method of enriching uranium, will begin operation by 1984.

The centrifuge plant can be expanded every two years to meet fresh demand if nuclear power continues to grow. By 1990, it could accomodate almost 30 million kilowatts of electricity all by itself.

Besides reopening the uranium order books, Schlesinger also described a plan to iron out what he said were "inequities" in oil entitlements. He said the New England states would be granted virtually full entitlements instead of the partial entitlements they new receive to import the residual oil used almost exclusively there to generate electricity.

New England had been getting an 80 percent subsidy to pay for the higher-priced imported oil; it will now receive 95 percent.

"Is this a boon to New England?" Schlesinger said. "In the sense that it represents the removal of a burden for New England it is a boon."

Schlesinger said he opposes any move to curtail oil production at the Elk Hills Naval Petroleum Reserve or to export Alaskan oil to Japan to relieve what is now a glut of oil on the West Coast.He said most of the glut is in residual oil, partly because California has imposed tougher sulfur standards on the residual fuel that can be burned.

"What I would propose," Schlesinger said, "is that we allow the export of residdual oilto relieve the problem."

Schlesinger said the natural gas pricing compromise approved by congressional energy conferenas, which eliminates the double pricing system for intrastate and interstate gas, would free up 2 trillion cubic feet of gas to move into intersate pipelines.

"That will save us 1 million barrels a day," Schlesinger said, "or almost $5 billion a year in imports."