The power of an air conditioning unit is represented by British Thermal Units per hour (BTU/h or simply BTU). In general, a 20-foot by 20-foot room requires 6,000 to10,000 BTU. For multiple rooms up to 800 square feet, a unit between 10,000 and 16,000 BTU is suitable.
Begin by calculating the square footage in your house. Measure all the rooms you would like to be cooled by multiplying the length by the width. Add these together for the total square footage for your house.
Begin with the base BTU range for your square footage. See the chart on InspectAPedia in the Resources section for a complete listing of base BTUs by square footage.
Depending on the room type and setup, you might need to add additional BTUs to the base BTUs. For rooms with an uninsulated roof or ceiling, add 4,000 BTUs. For kitchens, add 4,000 BTUs. For each window that receives sunshine throughout the day, add 1,500 BTUs. Also, add 600 BTUs for each person that will be occupying the room (e.g., a bedroom). Subtract 1,000 BTUs if the room is on the shaded side of the home.
The base BTUs plus the additional BTUs should give you a rough idea of what kind of air conditioning unit you need. However, there are a couple things you should keep in mind. Do not buy a bigger air conditioning unit than you need. Air conditioners that are too powerful for the space cool the air too quickly and will shut off before the air is dehumidified. Ultimately, this will cause the units to use more energy. You'll also need to adjust if your building gets an unusual amount of direct sunlight or is in a sunny region. High ceilings also require more cooling power.