PERHAPS, at last, the moment has arrived for

the energy tax. Congress has now committed itself to raising $21 billion a year in additional taxes, but which taxes? The Senate Finance Committee has given itself very little time to produce legislation; a bill is due on July 12.

If it were applied equally on all forms of energy, a $21 billion tax would increase the price of gasoline and fuel oil about 3 percent; on electricity, which is more expensive, it would be about 1.5 percent. The tax on all energy has the advantage of bearing equally on all uses without discrimination. But perhaps, you feel, that's also a disadvantage. Although most people squawk loudly at the thought of higher taxes on gasoline, most people would probably concede that home heating oil ranks higher on the scale of necessities.

Would you prefer to exempt home heating oil? Then you'd have to exempt electricity for home heating--and, the southern congressmen will immediately add, for home cooling as well. It would then be very difficult to collect any tax at all on electricity. This slippery slope soon leaves you with a tax on gasoline and not much else.

A tax of 22 cents a gallon on gasoline would raise $21 billion a year. Is that outrageous? Added onto present prices, it would return the cost of a gallon of unleaded regular gasoline to a level that, adjusted for inflation, would be slightly lower than the peak in the winter of 1981.

There's only one serious argument against an energy tax. Like all sales and excise taxes, it would be counted as a rise in prices by the Consumer Price Index. Since that would in turn raise all the wages and pensions that are linked, by law, contract or custom to the CPI, it would contribute to inflation. Perhaps the moment has also arrived to resolve that statistical peculiarity. If Congress passes the energy tax, it could usefully add a line dropping the tax out of the CPI. As more kinds of income are set by formulas based on the CPI, the case from leaving all taxes out of it becomes progressively stronger.

Most taxes serve only one purpose, to raise money, and they frequently do it at a considerable cost in inefficiencies and distortions imposed on the economy. An energy tax serves a double purpose, for it also pushes toward conservation and greater efficiency. It's long overdue.