ANYONE WITH a central air conditioner or heat pump who has mowed the lawn is familiar with the heat that blasts from the air conditioner fan. It usually hits you around the knees, if you wear shorts in the summer.

Good enough, you think. Get that heat out in the wide-open spaces where it belongs.

And so we go about throwing away good energy. If we harnessed that heat for something useful, such as heating hot water, we could cut by almost 1 percent our total annual energy consumption.

That's one quarter quad, estimates the Electric Power Research Institute, or a number of BTU's equal to a 10 followed by 14 zeros.

And the day when that happens may not be as far off as you might think. A number of companies already have introduced, or plan to introduce soon, gadgets that heat hot water with thermal energy we now throw away.

We are talking here about nothing short of revolution in hot water heaters.

The U.S. Department of Energy, the National Bureau of Standards and several dozen power companies across the country are investigating two devices that propose to cut water-heating bills in half or, in some cases, eliminate them entirely.

One is a cogenerator, commonly called a heat reclaimer, a device that takes waste heat from air conditioners and heat pumps and saves it for heating water. The other is a heatpump hot water heater. According to the manufacturers, both types can be installed for about one quarter the cost of a typical solar hot water system. Heat Reclaimers

The first is a simple heat exchanger. Carrier, the people who make air conditioners, recently introduced one called Hot Shot. A company called Energy Conservation Unlimited (ECU) in Orlando, Fla., is marketing another heat-saving device under its own name. (General Electric has picked up the ECU reclaimer and will market it with the GE label.)

Both Carrier and ECU estimate that the device, on the average, will supply at least 50 percent of a typical family's hot water annually, essentially cutting electric bills for hot water in half.

About 18 inches wide, 18 inches high and 4 inches deep, the heat reclaimer attaches to a heat pump or central air-conditioning system, as near as possible to the compressor. The hot exhaust line, which carries refrigerant at temperatures around 240 degrees, enters at one end. At the other end a line hooks into the hot water tank.

The heat exchanger inside the device absorbs heat from the compressor exhaust. With that it warms water from the hot water tank to 140 degrees and returns it at a rate of about nine gallons an hour.

Graham Harris, president of Energy Conservation Unlimited, said the firm has been selling heat reclaimers for nearly six years, mainly in the Sun Belt, where air-conditioning loads are greatest. About 50,000 already are in operation, he says, and these include several hundred at Disney World and in homes from Florida to Arizona.

Carrier puts the cost of equipment and installation for a Hot Shot at around $400 for a new house, $600 for a retrofit. ECU's estimates are somewhat lower: $400 to $500 for a retrofit, installed.

Several northern power companies regard both the heat reclaimer and the heat-pump hot water heater as a potential means of reducing peak loads. They believe that if the gadgets can save electricity in the South with air conditioners, they can do the same in the North with summer air conditioning and winter heat-pump heating. (Heat pumps, like air conditioners, exhaust heat during the cooling cycle. In winter, on the heating cycle, they can heat water more than twice as efficiently as the electrical resistence coils in a standard water tank.)

The Virginia Electric Power Company (VEPCO) is installing 40 heat reclaimers in new heat-pump-conditioned houses around Richmond, where annual water heating requirements are estimated to be 4,810 kilowatt hours a year for the average family of four. At 6.3 cents per kwh, that amounts to more than $300 a year.

Utilities in southern states that have seen reclaimers in action report they have measured up to expectations and usually pay for themselves in electric-bill savings within three or four years.

Officials of the Arizona Power Co., for instance, began testing them in 1976. They were attached to six homes with 52-gallon hot water tanks and air conditioners in the 3-ton range.

In a climate where daily summer temperatures hit 100 degrees and hover in the mid-80s at night, "the device was able to supply almost all water-heating requirements of those families, June through September," said Arizona Power's magazine of marketing services, Tom Morron.

Morron said about 500 units already are in use there, and all qualify for a $100 state energy tax credit. Several Arizona builders have started offering them as standard equipment. Not only will they cut customers' hot-water heating bills, which average $25 to $30 a month, he said, "they will almost totally elimate [the company's] hot-water-heater load during the summer months."

The Alabama Power Co. has experienced similar results.

In 1966, said the company's supervisor of residential energy services, Frank Denney, Alabama Power made a reclaiming device of its own and found it saved around 50 percent in hot-water-heating requirements over the year when attached to a heat pump. But, Denney said, "at that time electric rates here were only about 1 1/2 cents a kilowatt hour. Water heating bills were less than $50. So even though we could save 3,000 kwh year 'round, the savings weren't very much."

Snce then, however, electricity rates have climbed to 5 cents per kwh. Alabama Power went back and tested eight ECU reclaimers and confirmed its earlier results.Two years ago it began marketing the device and placed it under its 10-year heat-pump service contract.

"This year," Denney said, "we're expecting to connect about 500 units." Heat-Pump Water Heaters

Another device designed to save on hot water bills is the heat-pump hot water heater. Several companies are preparing to bring them on the market soon, both as units with integrated tanks and as retrofit equipment for standard hot water heaters. Utility officials who have seen them are confident they will radically reduce electricity requirements.

"We are expecting substantial energy savings," said Mitchell W. Shapiro, an associate engineer at the Baltimore Gas & Electric Co. "We're looking into the possibility of marketing it."

One such manufacturer is Energy Utilitization Systems in Pittsburgh.

The EUS model, being tested by Baltimore Gas in two homes there, will come with either a 52-, 66- or 82-gallon tank when the company begins mass marketing in May, said sales manager Debbie Kent. Drawing 800 watts of electricity, it delivers to 7,000 BTUs of power an hour, enough to heat 10 gallons of water to 140 degrees while using one-half to two-thirds less electricity than a standard resistence tank, Kent said.

In the Baltimore home of Gerry and Maureen Schonecker, for instance, BG&E placed the water heater in a small, quarter-basement utility room, where the heat pump uses excess heat from the nearby furnace as well as from the washer and dryer.

This year the EUS heater is expected to retail for around $600, mainly through utilities. When mass production begins, said Kent, the price should drop to $500 or less.

In Atlanta, Ga., a company called E-Tech is selling a mail-order retrofit heat pump for hot water heaters. Over the past two years, said company president Glen Robinson, more than 200 have been sold to utility companies across the country. The Tennessee Valley Authority has found they save, on the average, 50 percent of water heating costs. At Hawaiian Electric in Honolulu, employes were so impressed that several ordered them for their own homes.

The E-Tech heater weighs about 80 pounds. It sits on top of the regular tank and plugs into a standard 115-volt outlet. The 1-horsepower compressor draws 1,300 watts and delivers 12,000 BTUs to heat 19 gallons of water to 140 degrees each hour, Robinson said.

(It retails for $550 and can be ordered by writing E-Tech at 3570 Americana Drive, Atlanta, Ga., 30341. Unlike heat reclaimers, which require installation by a certified heating or cooling specialist, the retrofit heat pump is designed for installation by competent do-it-yourselfers.)

While the heat pump is heating the hot water heater, it draws warm air from the house and exhausts cool air. That might help keep houses cooler, and less humid, in the summer. But officials see the flip side as a potential drawback. The cooling action could make some rooms close to the water heater less comfortable than you're used to in the winter.

Kent said it could lower the temperature of a small utility room by 3 to 5 degrees. Consequently, EUS recommends its units be placed close to living areas in the South, where air-conditioning seasons are longest, and in basements in the North.