The gas company says its fuel is the greatest bargain. The electric company contends its prices now are lower than heating oil, and when a heat pump is used, lower than natural gas. The heating oil industry counters with numbers showing that its prices are less than electricity and increasingly competitive with natural gas.

Which claims should a homeowner believe?

If you're uncertain, you're not alone. In trying to determine the most economical way to heat their homes, consumers are faced with a bewildering array of claims and counterclaims, and variables such as equipment efficiency, fuel availability and local fuel prices. Sorting through it all to find the cheapest fuel can be confusing -- even for the experts.

At the federal Department of Energy, for example, officials only shake their heads when asked to name the cheapest fuel.

"We provide various statistics, but we don't have a specific answer for that question," said Bill Liggett, an analyst in the department's energy information administration.

And the statistics, it turns out, are not always so helpful either. "I could do an analysis coming up with any answer you want and do it without cheating," said Eric Hirst, a mechanical engineer who does research on energy conservation at the Oak Ridge National Laboratory in Oak Ridge, Tenn. "I don't think there is a best fuel. It's like asking which car should America be buying -- Chevy Chevettes, Ford Escorts or some other model. What we see is that some people buy some and some buy others."

Local energy officials who must deal daily with consumers seeking practical guidance on energy issues have worked out a general response for those who must decide now whether to keep their existing home heating system or convert to another kind.

"From a human point of view, our answer is to tell people that it isn't cost-effective to change their heating system from oil to gas because there is no clear-cut winner or loser -- except for electric resistance heating, which they should avoid, because it is very expensive," said Charles Clinton, director of the D.C. energy office.

That advice is in sharp contrast to what Clinton used to recommend.

"Several years ago, in 1979 or 1980, we would tell people that natural gas was the fuel to get if they could get it. But this year is different. Heating oil prices have stabilized and natural gas prices are bumping up due to deregulation and local rate increases. That has made prices so close that it is hard to pick one over the other."

In the meantime, the electric heat pump has emerged as an alternative to the oil and gas furnace from the standpoint of operating cost, Clinton said, though "only as long as the temperature is 30 degrees or warmer. If the temperature drops below 30 degrees, the more expensive resistance heat unit kicks in to supplement the heat pump."

Although the switch to natural gas has slowed recently, it is still the most popular source of heat in Washington-area homes. About 53 percent of the 1.1 million households in the area use it, compared with 31 percent that use heating oil and about 10 percent, electricity. The rest rely on other fuels, such as kerosene or coal or are unheated.

Consumers shopping for a house -- either a new one in which they choose the heating system or an older home with a heating system that may need replacing -- should have an energy analyst look at the house and diagnose its needs, Clinton said.

What is best for the house will depend on the construction material, the insulation, the shape and size and other factors that an analyst could include into the equation, he said.

In addition to the characteristics of an individual house, the search for definitive answers to heating fuel cost is complicated by several other factors -- some of which can be measured in dollars and some of which can't. These include:

* The method of computing costs. Is it appropriate to count only the fuel bill? Or is it more accurate to count the full cost of a heating system, starting with the purchase and installation cost of the equipment, and adding the maintenance cost that can be expected over the life of the equipment as well as the life-cycle operating cost?

There is general agreement that the most accurate way to compute heating costs is to consider the entire life cycle of the equipment, including the cost of buying and installing it, the cost of maintaining it and the cost of the fuel itself.

* The fluctuating price of fuel. Natural gas prices in October were 69 cents a therm; they will be 75 cents a therm in March. Electricity prices for Washington-area Pepco customers during October averaged 5.4 cents a kilowatt-hour for all-electric incremental heating, but the average within the jurisdictions ranged from 6.1 cents a kilowatt-hour in Virginia to 5.3 cents in Maryland and 4.9 cents in the District of Columbia.

But the District's average price will increase if Pepco wins its pending request for a rate increase, and that will push up the area average.

Meanwhile, average heating oil prices rose from $1.20 a gallon in October to $1.27 last month. Within the latest range, homeowners were paying from $1.22 to $1.29 a gallon.

* Dependability, flexibility and comfort of the heating systems. One disadvantage of the heat pump is that it doesn't provide adequate heat in extremely cold weather. As the outdoor temperature drops, the heating capacity of the heat pump decreases, so it can no longer satisfy the total heating requirements for a home, and an electric resistance heater typically switches on.

Furnace heating also has a disadvantage, however. The furnace system typically operates off one thermostat; to cut back on the heating load, the homeowner must block the registers on a room-by-room basis.

* Efficiency of the equipment. This varies enormously, from reported averages of 65 percent for some gas furnaces up to 92 percent for the new "pulse combustion" furnaces in the manufacturers' showrooms.

The percentage indicates how well the equipment uses fuel to provide heat; the higher the efficiency number, the less fuel needed to make one unit of heat and the lower the fuel cost for the homeowner. But manufacturer claims of high efficiency often outstrip what the homeowner actually will get in the way of performance.

In addition, some homeowners are more careful about keeping their equipment in top operating order so that they get maximum efficiency, than others who get only minimum efficiency -- just as some drivers get more miles per gallon from the same model car than other drivers.

The efficiency figures are but one weapon used by representatives of the three competing heating industries as they seek to maintain their share of the market, and if possible, increase it at the expense of the competition.

Pepco has been running television commercials in which a trio of young women sing about the "thrifty heat pump." Pepco also has published a new brochure comparing the cost of heat-pump operation with other types of fuels. And a heat-pump association formed by area contractors and a Pepco official is trying to promote the product.

Washington Gas Light, which traditionally has focused its advertising on gas appliances, now features the new, more efficient gas heating systems in its promotions.

And the heating oil trade association is planning a campaign calling attention to the advantages of heating oil.

In the rush to capture customers, the competing fuel industries often point to average costs that are based on optimum circumstances that the average consumer often cannot duplicate.

"They give their best case for everything -- and that's not realistic," said John Carlin, an energy specialist with the federal Department of Energy.

In the face of this flood of information, consumers are on their own to decide what is best for them.