Determine how many rooms the central air unit will be responsible for cooling.
Measure the square footage of the rooms and add them together to determine how many square feet the central air unit will be responsible for cooling.
Use the base BTU table (see resources) to determine the base number of BTUs that are needed. For example, for several rooms with a total area of 600 square feet your base BTU number would be between 13,500 and 15,000.
Adjust the base BTU number for room usage. If any room will be used for a kitchen, add 4,000 BTUs. Add 1,500 BTUs for any room above a kitchen. If more than two people will be occupying a room on a regular basis, add 600 BTUs for each additional person starting with the third.
Adjust BTUs for structure. Any room that is not below an insulated roof adds 4,000 BTUs. If a room receives significant sunshine, add 1,500 BTUs and if it is well shaded subtract 1,000 BTUs.
- If you do not have your new system professionally installed, the U.S. Department of Energy estimates that it can lose up to 30 percent of its efficiency.