One common misconception is that “Bigger is Always Better”. When it comes to air conditioning that is definitely not the case. Bigger is actually much worse in many ways. A lot of people think that a bigger unit will run less and cost them less on their electric bill. It is true that it will run less, but that does not cost less. The bigger unit will start and stop more, which is very inefficient. It takes approximately 8 to 10 minutes of run time for an air conditioner to get up to its full efficiency. If the system starts and stops it is like driving your car in stop and go traffic, versus driving on the highway, you get much worse mileage in stop and go traffic. Just the same your air conditioner is much less efficient with more cycling.
The other disadvantage of the starting and stopping is that the unit does not really start dehumidifying until it has been running for about 8 to 10 minutes. You get much worse dehumidification from an oversized unit and then you have a tendency to turn the thermostat much colder to try and get comfortable. Every degree colder you turn your thermostat costs you 10 to 15% more energy, which makes the larger unit cost you much more to run. See The AC-Enhancer Info.
It is critical to get the sizing correct when you are replacing your air conditioner, it is something that you typically only do once every 10 to 15 years, and you don’t want to be stuck with the wrong size for that long. A simple rules of thumb is if during the hottest part of the summer and the heat of the day, your unit cycles off, then you have some excess capacity. A properly sized unit should run 100% of the time when it is 96 degrees outside and 76 degrees inside. Once it starts cooling down outside then it will start to cycle off some.
Sizing a unit for a home properly takes many factors into consideration such as, square footage, ceiling heights, windows, doors, attic and wall insulation, infiltration or leakiness, solar loads, appliances, and people loads. There is no square foot per ton method that is accurate because of all the other factors that can vary so much. Also, if you have made energy upgrades to your home such as new windows, extra insulation, weatherstripping, or anything else you will decrease the size of air conditioner required to cool your home.
Allowing the home to warm up a degree or two during the hottest part of the hottest day of the year is much better than having to pay 10 to 15% extra on your cooling bill for the rest of the year. If we size our units for design conditions we are actually sizing for an average of 4 hours a year. That is why when we oversize units it is drastically worse. If you are considering replacing your air conditioner you may want to look at reducing the tonnage a little, but be very cautious of increasing the size.