Oversized air conditioners are very common, especially in energy efficient homes. Rule of thumb sizing is often used, which is designed to have ample capacity to compensate for duct leaks and other common problems. Systems should be sized according to a "Manual J" calculation.
Oversizing reduces "call-backs" for the contractor. Selling a bigger, more expensive unit may also be more profitable. The homeowner loses on every front (higher price, higher energy costs, less comfort due to poor dehumidification, shorter equipment life). The U.S. Department of Energy has more information on sizing heating and cooling systems .
