The cooling capacity of an air conditioner is measured in tons. In regards to cooling, 1 ton is equal to 12,000 BTU's (British Thermal Units.) A standard air conditioner ranges in size, from smallest to largest:
For example, a model SSX160241 is a 2 Ton (24,000 BTU) air conditioner.
There is only one correctly sized air conditioner or heat pump for your home. A unit that is oversized will turn on and off too frequently, called “short cycling.” Short cycling causes the unit to lose both efficiency and the ability to properly dehumidify the home. This may result in higher utility bills and a less comfortable home. A unit that is undersized will run too often, and on the hottest days of the year, may not be able to adequately cool the home.
Additional Sizing Methods
There are other ways to calculate the correctly sized furnace for your home.
Manual J Load Calculation
This is the proper and scientific method for calculating AC size taught to HVAC technicians and recommended for use by professionals in the trade. It consists of taking information about your home’s construction materials, insulation levels, number of windows, sizes of rooms, etc, and making a calculation based on those factors to determine the appropriate heating and cooling requirements needed.
Does your neighbor have the same size home as you? If he has a properly sized air conditioner, then the same size unit may also work for you.
Ask a Contractor
Most air conditioning professionals give free in-home estimates for installing new air conditioning, during which they will recommend a unit size. While we don't encourage using contractors for quotes if you don't intend to hire them, a contractor familiar with the homes in your neighborhood will likely be able to give you an idea over the phone of what size you might need.
If the unit you have now is the correct size for your home, replace it with the same size. How do you know what size you currently have? Look at the name plate on the outdoor condensing unit and locate the model number (not serial number). You are looking for 2 digits in the model number that match the numbers below to indicate Tons or BTU.