Regardless of the level of efficiency of an air conditioning unit, ensuring that you pick the right size for your needs helps you to achieve the balance between energy consumption and optimal comfort levels. There are many factors that play into the picking of your AC unit, and below is a discussion of some of them:

The condenser-evaporator balance

The right AC size achieves a proper balance between the condenser, which pressurizes and cools the refrigerant and the evaporator, which cools and dehumidifies indoor air before transferring it to the air handler for circulation within the house.

Condenser sizes are expressed in refrigeration tons, where one ton is equivalent to 12,000 BTU per hour – describes the amount of refrigeration required to freeze a ton of water within 24 hours. In simpler terms, the British Thermal Unit tells you the efficacy of a unit in cooling a space. Condenser sizes must be recommended based on insulation of the home, general climate (which determines the temperature difference needed), air leakage and exposure to direct sunlight.

Check model number

The best way to determine the correct size is to look at the unit that is currently installed. The capacity of your air conditioner is described within the model number of your outdoor unit. Manufacturers have almost standardised model coding. For example, the '18' in the model CKL18-1 describes that the unit has a capacity of 18,000 BTUs. Given that there are 12,000 BTUs per ton, that model is a 1.5 ton AC unit. In the same way, a CKL24-1 model is a two-ton unit (24,000 BTUs).

If your current system serves you efficiently, you should pick a unit of similar capacity. If not, talk to your HVAC service specialist about increasing or decreasing unit size. The following special considerations should be applied when installing units for various rooms:

  • For a room that receives excessive, direct sunlight (such as those with west-facing doors and/or windows), increase the required BTU by 10 percent
  • For a kitchen AC unit, increase the required BTU by 4,000 BTUs to correct for the higher kitchen temperatures because of cooking
  • Rooms that seat more than two people on a regular basis should have 600 BTUs per extra person over the required BTU

The required BTU is determined by the room size – larger rooms should have units with more BTUs and vice versa.

Importance of optimal AC sizing

Getting larger AC units doesn't mean that your room will be cooled more efficiently. If you install a unit larger than the space being cooled, the system will go into frequent short cycles. This means that the air handler/compressor isn't kept running long enough for proper air dehumidification, reducing your comfort in the space. This happens because the room is cooled down too quickly, then the thermostat shuts off the air handler before it has sufficiently circulated dehumidified indoor air.

In addition, the larger unit consumes more energy less efficiently, reducing its lifespan and increasing chances of breakdown. Similarly, a smaller AC unit than you need overworks the system, causing the condenser to be clogged with frost and shut down periodically. However, this is a less common problem since suppliers tend to round up your requirements to reduce complaints about inefficiency.