The technical description is: "a load calculation is a method of determining the heat gain and loss of a home or building so that HVAC equipment is properly sized. A building's heating or cooling design load is based on how well insulated the building is and in what climate it is located.”

Problems that arise from over-sizing a system are as follows:

During the air conditioning season, a main characteristic of central air is not just bringing the temperature down to get to the set/desired temperature, but also removing the humidity in the home/structure; the ideal percent of humidity should be between 30-50%.

When you have an oversized system, it will struggle to maintain the 50% dehumidification, and you can accomplish bringing the temperature down; however, if the humidity is above 50% your body will feel uncomfortable and in turn, you will then lower the thermostat temperature, trying to compensate for the over humidification of the home/structure. So, the end result is to focus on removing humidity and the key is sizing the unit correctly.

Another problem from oversizing a system is short-cycling the compressor. To give an example, when driving a vehicle do you get better gas mileage in town or on a highway (driving a gas-powered vehicle)? Most understand you get better gas mileage on a highway due to no starting and stopping, as there is in town. In the same way, that is how a central air conditioning unit operates, more efficiently and effectively, with a lower utility bill.

Another side-effect of oversizing is getting less longevity out of an oversized unit, with more wear & tear on the system and higher utility bills. And finally, an oversized system can create an environment for mold to thrive. With high moisture content and darkness, these are both contributors to growing mold.