PRESIDENT CARTER is about to get himself - and the country into serious trouble with his silly promise to balance the federal budget by 1981. His Office of Management and Budget is now quarreling with the department of Energy over the size of the government's strategic petroleum reserve. Last summer the Department started buying oil and pumping it into undergroung caverns against future emergencies. The issue now is the size of the reserve that the country needs - whether it should be 500 million barrels or 1 billion barrels.

There's no simple rule of thumb to answer that question. When Congress established the reserve in 1975, it authorized the President to store "up to" 1 billion barrels of oil. President Ford set a more explicit target of 500 million barrels by 1982. Last spring President Carter speeded up the schedule, calling for 500 million barrels stored by 1980, and another 500 million by 1985. This progression of goals accurately reflected the rising sense of urgency within two administrations, as the scale of American oil imports steadily rose.

When Congress passed the original law just two years ago, this country was importing 1.8 million barrels a day from the Arab countries. Today the figure is a little over twice that. In 1975, 500 million barrels would have been a nine months' supply against another Arab embargo. It has now become enough for only four and a half months - not quite as long as the 1973-74 Arab embargo actually lasted. But another embargo by the Arab countries is hardly the only threat.

In the 1975 legislation Congress spoke of a reserve equal to 90 days' import from all sources. That's a good conservative measure. Under today's circumstances that would require 800 million barrels in storage - and the trend in imports is, of course, steadily rising. Americans also need to remember that this country has committed itself to share its own oil with the other industrial nations in the event of a world-wide cutoff - the kind of catastrophe that might follow, say, warfare in the Persian Gulf. A billion barrels of oil sounds like a monstrous amount. But the economic stability of the United States, and of every other industrial country, now depends upon maintaining a steady flow.

The money for the first 500 million barrels is in hand. The quarrel between the budgeteers and the Energy Department is over the second 500 million barrels and , specifically, a sum of $261 million in the forthcoming 1979 budget to begin building facilities. The OMB perceives, correctly, that this appropriation would be a commitment to much larger amounts in succeeding years. The budget-makers are already throwing out items that might jeopardieze the President's famous pledge about the 1981 budgets, which goes into effect five weeks before the next presidential election.

It's possible to argue that 500 million barrels is enough for the first phase of the reserve and that, when the country gets there, it can pause to assess its prospects and decide then whether to go further. That's a seductive logic for the White House, since it would mean that in one year's budget - as you guessed, fiscal 1981 - there would need to be no money for oil reserves. But that strategy violates the whole principle on which the Carter energy policy is built.

As Mr. Carter has repeatedly said, his administration strongly believes that worldwide oil supplies aregoing to get suddenly much more expensive and more difficult to expand in the 1980s. If that's right, it makes no sense at all to postpone filling the reserve. One billion barrels is certainly not too large an amount, and Mr. Carter cannot retreat from his goal now without casting his whole energy plan into still deeper jeopardy. It's going to be expensive, but oil in storage is, you might say, money in the bank.