Saving energy doesn't have to mean sacrificing comfort when it comes to keeping cool.

President Carter recently signed an order requiring thermostats in non-residential buildings to be set at a minimum of 78 degrees Fahrenheit during the summer. The government is urging homeowners to keep the same temperature level on a voluntary basis.

You can turn up the thermostat without sweltering if you plan ahead to make the most efficient use of electricity. Start by keeping energy in mind when you shop for and use an air conditioner.

Do not buy a model that provides more cooling than you need; it will not dehumidify properly and you will waste money.

Cooling capacity is measured in British thermal units per hour. (One Btu represents about the amount of heat produced by burning a wooden kitchen match to ashes.) A 5,000-Btu air conditioner, therefore, will remove approximately 5,000 Btu of heat from a room every hour.

Air conditioning experts generally say a 5,000-Btu unit is adequate for a small bedroom; you'll probably want double that cooling capacity for a family room or den.

Individual needs vary, however. A room that is used primarily in the evening, for example, takes less cooling than one that is occupied during the heat of the day. Check family habits before you buy.

Compare Energy Efficiency Ratios to see which model gives you the most cooling for the least electricity. To find the efficiency ratio - also called the EER - divide the number of Btus by the number of watts. The higher the EER, the better. (Many models list the EER; if the model you are considering does not include the figure on its label, you'll have to do the calculation yourself.)

Check the wiring before you plug in your air conditioner. Most homes use 115-volt, 15-ampere circuits for lighting and small appliances. Some models can be plugged into the outlets of these circuits; others require separate wiring. As a general rule, a special circuit must be installed for any 115-volt air conditioner that requires more than 12 amps or for any 230-volt unit.

If your air conditioning is connected to a thermostat, set the temperature at 78 and leave it there. Do not try to speed up cooling by turning down the thermostat when you turn on the air conditioner. It won't work.

Set the fan speed on high except in very humid weather. When it's humid, set the speed at low; you will get less cooling, but more moisture will be removed from the air.

Clean or replace filters at least once a month. When the filter is dirty, the fan has to run longer to move the same amount of air.

Turn off your window air conditioner when you leave a room for several hours. You will use less energy re-cooling the room then you would if you left the unit running.

When possible, don't use the air conditioner at all. Keep shades, venetian blinds and draperies closed when the sun is shining and open them when the heat eases.

According to a study conducted by the Illinois Institute of Technology for the window shade industry, between 40 cents and $1.40 is added to fuel and utility bills every year for every square foot of single-pane glass in a house.

The institute study also showed that when the outside temperature was 85 to 95 degrees a light-colored, opaque or translucent window shade cut the amount of heat admitted by more than 40 percent.

Consider installing a whole-house fan. This type of fan, sometimes known as an attic fan, pulls outside air through the house and exhausts the warmed air through the attic. The moving air makes living areas feel cooler.

According to the American Ventilation Association, most people can be comfortable at a temperature of 90 degrees if the humidity is 50 percent or less and there is a gentle breeze of three to four miles per hour.

Do not use your air conditioner and whole house fan at the same time; you will simply draw cold air up and out through the exhaust system. As a general rule, it's a good idea to use the fan instead of the air conditioner when the outside temperature is 85 degrees or less.