A good load calculation program takes into account window types, window orientation, window shading, insulation of ceiling, walls and floor, air leakage, and many other factors, such as the color of the roof and the number of occupants. Using the square footage of a home to size an air conditioner is outdated and will almost always yield an oversized system. Don’t use a contractor who wants to size your unit solely on the square footage of your house. Require a Manual J or equivalent analysis before purchasing.