IN THE next few months, new mandatory government labels are expected to appear on the marketplace showing how much energy is consumed by major appliances. By disclosing operating costs in dollar amounts, they are intended to enable consumers to shop around for the most energy efficient models.

Developed by the Federal Trade Commission (FTC) and the National Bureau of Standards NBS), the labels will tell consumers how much energy the appliances they buy will use in the course of a year. The program covers refrigerators and refrigerator/freezers, freezers, dishwashers, water heaters, room air conditioners, clothes washers, central air conditioners and furnaces.

In most cases the labels show in dollar amounts how particular products compare in energy consumption to competing models, as well as adjusting estimates to different utility rates across the country.

The labels on 23 cubic-foot refrigerator/freezers for example, might show the energy cost spread as $68 for the least hungry model, $132 for the biggest electricity gobbler. The label on the imaginary reffrigerator we are considering estimates this one will use $91 in electricity annually.

To localize that cost, look at the "cost per kilowatt hour" chart on the label, which tells you the same machine will cost about $44 a year where the rate for electricity is 2 cents per hour, $88 where it is 4 cents, $132 where it is 6, $176 where it is 8 and $220 where the price is 10 cents per hour.

If your local electricity rate is 12 cents an hour, you can expect to shell out a hefty $264 a year to operate this refrigerator.

The label is not the same on all types of appliances. On air conditioners, for instance, it compares EERs (Energy Efficiency Ratios), the measure of how many BTUs in cooling the machines put out in relation to the amount of electricity they use. A chart shows the expected annual operating costs of different sized models.

And because operating conditions vary so much from house to house, labels on furnaces do not say how much they will cost to run. Instead they indicate which are the most efficient and refer the buyer to a handbook that computes costs according to several variables.

Although in developing the labels the NBS attempted neither to test all appliances not to compare different brands, it did find that comparable makes of some appliances can vary greatly in the amount of energy they use.

Some newer water heaters, for instance, are built with twice the amount of insulation as conventional models. Figures show that these consume as much as 12 percent less energy.

Methods of using insulation, and other features, such as an on/off switch to control the anti-condensation mechanism, can also affect the performance of different refrigerators. Foam, for example, insulates better, inch for inch, than conventional Fiberglass.

Some manufacturers switched to foam to increase refrigerator capacity. Many are returning to the old capacities and using more foam. As a result, some refrigerators are more efficient.

In some cases, said Andrew J. Fowell, chief of the bureau's product performance engineering division, the difference in energy consumption between comparable brands can be as much as two to one, with possibly hundreds of dollars in electric bills in the balance.

"Which means," Fowell said, "that it might be worth it to pay an extra $50 for the more efficient of the two."

Fowell believes the new labels could change American appliance buying habits and that industry, in turn, may be forced to build better machinery. (The NBS is currently working on minimum standards for the Department of Energy.)

"I think you will see a definite trend toward better efficiency."