One of the questions most often ask is what size air conditioner do I need to heat cool a 3000 square foot house? The problem is you are asking the wrong question. The question should be how much heat gets into my 3000 square foot house that the AC System must remove.
We could tell you that the average home needs one ton of air conditioning for each 500 square feet of conditioned space and we could tell you that the average man wears a size 7¼ hat. If you buy yourself a 7¼ hat without measuring, it will probably be the wrong size.
Heat does enter the building through walls and ceilings, so there is a relationship between AC size and square feet. The amount of heat entering through the ceiling and other building envelope components is highly variable depending on factors such as the amount of insulation in the attic.
There are many sources of heat that must be measured or estimated to determine the AC System size. They include: walls, ceilings, windows, floors, doors, air leaks as well as internal heat such as people and kitchen appliances.
Other factors are also important to calculate heat gain such as the outdoor temperature on the hottest days and the temperature you want your AC System to maintain indoors on those days.
You can hire an engineer to measure and calculate heat gain for sizing an air conditioner or heating system and many HVAC Companies will provide this service for no charge. There is no reason to guess. See this article for additional information