AS if taken utterly by surprise, residents flocked to home improvement centers and appliance stores last weekend in a frenzied effort to ride out summer's first wave of heat and humidity in the cool breeze of a brand new air-conditioner. In one large store in Secaucus, N.J., for example, sweaty Sunday shoppers picked over the remains of what was left on the shelves while a store employee proudly observed that on the preceding day more than $68,000 in air-conditioners went out the door.Experts in home cooling say they are hardly surprised by the mad dash to the appliance stores. What does surprise them is that so many people who buy air-conditioners end up with the wrong size and those who already have them fail to use them efficiently.''One of the biggest mistakes people make is not buying the right-sized unit,'' said Marc Rosenbaum, a professional engineer and a principal of Energysmith, an engineering company based in Meridan, N.H. Mr. Rosenbaum says the most significant factor used to determine the appropriate size for an air-conditioner -- whether a room air-conditioner or a central air-conditioning system -- is solar heat gain through windows.

And that, he said, is rarely taken into account during the purchase.''Air-conditioners for houses can't be sized by rules of thumb,'' he said, adding that with two rooms of equal size, one room could be three or four times more difficult to cool than the other if the first room has significant solar heat.Still, Mr. Rosenbaum said, most people shopping for air-conditioners determine the size of the unit they need by checking charts in stores or by consulting with retail sales representatives. Neither strategy, however, offers any guarantee of success.For example, the Carrier Corporation, an air-conditioner manufacturer based in Syracuse, recommends an 8,500 B.T.U. air-conditioner for a 400-square-foot room. The company suggests increasing the capacity by 10 percent for sunny rooms and decreasing it by the same amount for heavily shaded rooms.In addition, the company suggests adding 600 B.T.U.'s for each additional person if an area is routinely occupied by more than two people and by 4,000 B.T.U.'s if the room to be cooled is a kitchen.

Isaac Ettedgui, vice president of the Five Boro Air Conditioning, Heating and Refrigeration Corporation in Manhattan, had an easier way of figuring the appropriate size for a room air-conditioner.''types of ac distribution systemYou multiply the length of the room by the width to get the area and you multiply that by 40,'' Mr. Ettedgui said. how much is a brand new ac unitSo, for example, a room that is 20 by 20 feet would be 400 square feet multiplied by 40. inside ac unit will not turn offThe resulting number -- 16,000 -- is the B.T.U. capacity necessary for that size room.''If the room is a kitchen or has a ceiling more than eight feet high, you multiply by 45,'' he said.Without accounting for kitchens or high ceilings -- or sunny or shady windows -- Mr. Ettedgui's calculation results in almost twice as much air-conditioner as Carrier would recommend for the same sized room.

It's better to be safe than sorry,'' Mr. Ettedgui said. ''You're always better off getting something bigger than you need.''Though many may share that philosophy, it may not always work.''When it comes to cooling, bigger isn't always better,'' said John Proctor, managing partner of the Proctor Engineering Group, a San Rafael, Calif., engineering company that specializes in climate control. ''But most salespeople err on the side of caution.'' Mr. Proctor said that people who bought air-conditioners larger than necessary would end up paying more for their comfort -- and feel less comfortable -- in the long run.''When an air-conditioner first starts up it's very inefficient and removes very little moisture,'' Mr. Proctor explained. ''So when it's hot and humid and really icky, a big air-conditioner in a small room cools the room quickly but removes very little humidity.'' Cooling the room too quickly, Mr. Proctor said, causes the air-conditioner to cycle on and off more frequently than it is supposed to while removing less humidity than it should.

As a result, he said, people in the room end up feeling ''clammy and icky'' rather than cool and dry. Then, to make matters worse, people lower the thermostat to a cooler setting in an effort to make the room more comfortable.''But the thermostat is only measuring temperature, not humidity,'' Mr. Proctor pointed out, adding that the smartest thing to do with a thermostat is to set it where it provides the most consistent comfort and leave it there. Setting an oversized machine to a cooler setting only increases the frequency of the on-off cycling of the compressor while doing little to make the occupants of the room feel more comfortable.And that wastes both energy and money.Nadav Malin, editor of the Environmental Building News, an environmental newsletter published in Brattleboro, Vt., explained that the energy efficiency ratings for air-conditioners -- the numbers on the yellow tags that rate the relative costs of operating different units -- were based upon an optimally sized unit running constantly in an optimally sized room.''

But when an air-conditioner is cycling on and off, or running at partial loads, its efficiency goes through the floor and you can forget about the energy efficiency rating,'' Mr. Malin said.In fact, the efficiency loss of an oversized air-conditioner is so significant that some experts say erring on the side of caution means buying a unit smaller than needed.''I usually recommend buying a slightly undersized air-conditioner,'' said Sol Schwartz, an engineer and principal in the firm of T/S Associates in Manhattan. Since a smaller unit will require longer run-cycles, Mr. Schwartz said, the effective removal of humidity is assured.Mr. Proctor of Proctor Engineering, agreed. In fact, he said, the significance of avoiding an oversized system increases with the size of the area to be cooled. ''Central air systems are notoriously oversized,'' Mr. Proctor said. ''And that can make them notoriously inefficient.''If store charts, formulas and rules of thumb are inadequate for accurately determining air-conditioning requirements, what is a consumer to do?''

Manufacturers' recommendations are usually more accurate than store charts or formulas,'' Mr. Proctor said, adding that optimally sizing central air-conditioning systems usually takes a lot more effort.''There's a standard methodology for figuring out the appropriate size of a central air-conditioning system,'' Mr. Proctor said, explaining that engineering protocols take into consideration solar heat gain, insulation, window size and exposure and occupancy levels along with square footage.While it is possible to hire a professional engineer to conduct such an evaluation, homeowners thinking about having central air-conditioning installed are usually just as well served by getting estimates of size and price from local contractors.Dennis Costello, president of All Seasons Air Conditioning in West Babylon, L.I., said that at least three estimates should be obtained.''If you have three good contractors, all the estimates should be within about 6,000 B.T.U.'s of one another,'' Mr. Costello said.