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Properly Sized Room Air Conditioners

Many people buy an air conditioner that is too large, thinking it will provide better cooling. However, an oversized air conditioner is actually less effective and wastes energy at the same time. Buying the right size of air conditioner is in your best interest; only the right size of unit or system will efficiently and effectively keep you and your family comfortable. Air conditioners that are too big use more electricity and leave the air in your house with excess humidity. Air conditioners that are too small do not cool your home to a comfortable temperature. Choosing the right size air conditioner requires some basic math and just a few minutes of your time. Air conditioners remove both heat and humidity from the air. If the unit is too large, it will cool the room quickly, but only remove some of the humidity. This leaves the room with a damp feeling. A properly sized unit will remove humidity effectively as it cools. A good contractor will determine the optimum size for your home by making a careful study of your cooling requirements. Window dimensions and exposure. Floor space, insulation and local climate. Heat-generating appliances. The direction your home faces. Even the amount of your home’s exterior shaded by trees. He’ll specify the cooling capacity of the system in either Btu/h (British thermal units of heat removed per hour) or refrigeration tons (one ton being equal to 12,000 Btu/h).


Much like automobile manufacturers, today’s air conditioner manufacturers are required by law to evaluate and rate their equipment according to its energy efficiency. This rating is known in the industry as a SEER, or Seasonal Energy Efficiency Rating. The higher the SEER, the more efficient the equipment.

Most new homes with central air come equipped with a standard builder’s model. However, when replacement becomes necessary, property owners can upgrade their air conditioning by specifying a more energy-efficient system.

High SEER models are generally more expensive, but can easily make up the difference by reducing your home energy bills over the long run.

To figure out which size unit is best for your cooling needs:

  1. Measure the square footage of the room that you will air condition. Use a tape measure to measure the length and width of the room. Write these measurements on your paper. If the room is not square or rectangular, divide the room into sections composed of squares and triangles and take those measurements.

  2. Determine the cooling capacity needed for the room or your home. Cooling capacity for room air conditioners and central air conditioning systems is measured in British Thermal Units (BTUs) per hour. The United States Department of Energy Energy Star website offers these guidelines for BTUs per square footage: 100 up to 300 sq. feet: 5,000 to 7,000 BTU 300 up to 550 sq. feet: 8,000 to 12,000 BTU 550 up to 1,000 sq. feet: 14,000 to 18,000 BTU 1,000 up to 1,200 sq. feet: 21,000 to 24,000 BTU 1,500 up to 2,000 sq. feet: 30,000 BTU 2,000 up to 2,500 sq. feet: 34,000 BTU

  3. But you have many variables and factors to consider. For instance:
  • Height of the ceiling.
  • Number of windows and quality of the windows.
  • Sun Factor.
  • Room Insulation.
  • Type of room and maximum occupancy.
  • Geographical location.
  • Number of fan in ths room/house (if any)

We have a BTU calculator for reference only that will give you an idea of how many btu's you need for each room. For an accurate sizing please consult with your local HVAC contractor, they will help you determine the right size based on several variable factors.

Click here for btu calculator