- Determine the square footage of the area to be cooled. You can do this either by looking up your home's floor plan or by measuring the area of each room by multiplying the length times the width and taking the sum of all the rooms.
- Determine the base Btu requirements by using a sizing chart (see Resources). For example, a home with 1,700 square feet would have a base of 25,500.
- Add to the base for very sunny rooms. For each sunny room, calculate the area and then determine the Btu base for that room by using a room sizing chart and then calculate 10 percent of that number. For example, a 100-square-foot room would need 5,000 Btu, so you would add 500 Btu to your value found in Step 2.
- Subtract from the base for heavily shaded rooms. For each heavily shaded room, calculate the area and then determine the Btu base for that room by using a room sizing chart and then calculate 10 percent of that number. For example, a 375 square foot room would need 9,000 Btu, so you would subtract 900 Btu from your value found in Step 2.
- Add 4,000 Btu for each kitchen or uninsulated room in the house to the base found in Step 2.
How to Size Air Conditioners
Choosing the right size air conditioner is important both for your personal comfort and for your energy costs. To determine what size you need, you need to know how large the areas are that will be cooled and and how they will be used. The size of an air conditioner is determined by its capacity which is measured in British thermal units, or Btu. If your air conditioner is not large enough, it will wear out from trying to overwork.
- Getting an air conditioner that is too large is also detrimental. If your air conditioner is sized too large it will waste energy from turning itself on and off frequently.