Calculations for Electric Air Conditioners

Capacity

• The capacity of electric air conditioners is generally rated in British thermal units, or BTUs. One BTU represents the energy needed to raise the temperature of a pound of water by one degree Fahrenheit at sea level. A BTU is equal to about 0.293 watt-hours. An electric room air conditioner might be rated at 10,000 BTUs, for example. Another important factor is the operating cost compared to other models. This is expressed as the EER, or Energy Efficiency Ratio, for window air conditioners and as the SEER, or Seasonal Energy Efficiency Ratio, for central air conditioners. A higher rating means better efficiency and lower operating costs. BTUs, EERs and SEERs are normally listed on a unit’s label or in its specifications.

Window

• Estimating window air conditioning BTUs is fairly straightforward, since the unit only generally cools one space. First, multiply the width of a room by its length to determine the total square footage. For example, a room measuring10 feet wide by 20 feet long has 200 square feet. Rooms measuring 150 to 250 square feet need 6,000 BTUs. For 100 to 150 square feet, it’s 5,000 BTUs; for 250 to 300, it’s 7,000 BTUs; for 300 to 350, it’s 8,000; for 400 to 450, it’s 10,000; for 450 to 550, it’s 12,000 BTUs; for 700 to 1,000, it’s 18,000 BTUs; and for 1,000 to 1,400, it’s 24,000. Heavily shaded rooms reduce capacity by 10 percent, while sunny ones increase capacity by 10 percent. Add 600 BTUs for each additional person over two people who regularly uses the room, and if the room is a kitchen, the capacity goes up by 4,000 BTUs.

Central

• For central air conditioners, the calculations become more complex, since you’re dealing with larger units and spaces that are divided into smaller areas. Square footage is still important, but you cannot simply use the entire area of a house for this calculation. Instead, estimate the total square footage only for rooms that you want cooled, such as the living room, dining room and bedrooms. You must exclude non-cooled spaces such as pantries and closets, which are normally part of square footage.

Estimates

• Estimates begin with the tonnage of a unit to determine BTUs. Each ton usually supplies 12,000 BTUs, so a 1.5-ton unit has 18,000 BTUs of capacity, while a 3-ton unit reaches 36,000 BTUs. You can then estimate the tonnage needed based on the climate. In warm climate areas such as Texas and Florida, 1.5 tons cool 600 to 900 square feet; 2 tons manage 901 to 1,200 square feet; 3 tons work 1,501 to 1,800 square feet; and 4 tons handle 2,101 to 2,400 square feet. In the moderate climate zones, which include New York and Missouri, 1.5 tons cool 600 to 1,000 square feet; 2 tons manage 1,001 to 1,300 square feet; 3 tons work 1,601 to 1,900 square feet; and 4 tons handle 2,201 to 2,600 square feet. In the coldest climates of states such as Maine and Montana, 1.5 tons cool 700 to 1,100 square feet; 2 tons manage 1,101 to 1,400 square feet; 3 tons work 1,651 to 2,100 square feet; and 4 tons handle 2,301 to 2,700 square feet.